Under the World tab where you first create a new interior cell, you will see a series of color boxes for Ambiance, Sunlight and Fog. If you simply want a room to blend in with other similar rooms i.e. a Balmora interior, take a look at a neighboring property and jot down the numbers e.g. Ambient 75 65 65 Sunlight 80 60 20 Fog 0 0 0 density 0.75, and apply them to your new cell. If you want to create a special effect i.e. having a red or blue tinge to the light, useful if you are making a Morag Tong guildhall or underwater palace, just play around with the light settings until you find a color you like.
Avoid Dirty Mods
When editing references, do not click on the Save button! If you click the save button, not just the reference, but also the object type will be marked as changed. I.e., you'll have a Dirty Mod. Instead, click the cancel button or the close box.
On the other hand, if you want to replace an existing reference with a newly defined object (e.g., by changing the contents of a chest), then you might change the object id and then click save. The point is avoid making the change by accident.
All parts of an object are rendered whether visible in your interior cell or not, so if you have a large number of unseen polys outside of walls, your dungeon is going to chug on average systems even if the visible interior is very simplistic and smooth. It seems that even if only 5 polys (polygons) of a 180 poly rock are showing in the interior of a level, that entire 180 poly rock is still being rendered by the game engine as if it were wholly visible.
Most of the strain of rendering cells is taken not by the graphics card, but by the CPU of the computer (note: this is reversed by the fan-made exterior add-on program Morrowind Graphics Enhancer or MGE ). Consequently, even very high-end computers will experience FPS slowdown (often called "lag") in highly detailed cells. (Users of MGE may notice some improvement in this respect.)
Many of the newer Morrowind mods use more detailed models and textures, and these themselves can contribute to FPS slowdown. It is advisable to avoid putting more than 600,000 polys (faces) in a cell, or more than around 1100 objects. No individual object should have more than 50,000 faces. Even at these maximum settings, some slowdown will occur, but the average player is still likely to enjoy 15FPS (frames per second) or higher. This is lower than the industry-standard minimum of 30FPS for games, but widely seen as an acceptable compromise in an aging game regularly pushed to its engine limit. The absolute minimum FPS is around 9 FPS, which will still be frustrating for most players who may want to avoid that cell. If you cannot remove any more from your cell without compromising it artistically, it is best to factor in alternative journey-paths so that the player may avoid that cell in regular play.
Other performance-hitting factors are NPCs (non-playing characters) - especially those with added animation files such as dances; scripts; dynamic lights (flames); and sounds. Scripts should be 'optimized' to prevent them running every frame where possible. There are often two options to achieve each effect - such as being able to exchange a non-dynamic light for a dynamic one, or a low-poly plant (such as heather) for a high one (Timsa-Come-By flowers).
As you've probably noticed, a lot of the objects don't come in the sizes you first saw them in somewhere else in the game when dropped straight into the rendering window. All objects in game are "references". This means that they are all references to (or based on, if you will) an actual object in the main .esm file. You can double click a selected static and change anything below the Reference Data section in the pop-up window. This will only affect that particular selected static and will leave all other references to that object untouched.
To resize a static to better fit your specific interior needs, you simply change the value of 3D Scale on its pop-up sheet. The default 1.00 is equal to standard size. (100%). Statics can be effectively scaled down to 50% of their standard size by entering 0.5 in this field and scaled up to 200% of their standard size by entering 2.0 Any size in between those seems to work decently, but those seem to be the upper and lower limits, for reference.
You can however use a script to set an objects size outside the bounds of the normal scaling done in the editor. And a script is required to keep an item at a modified size if you can pick up the object. As when you put it back down it reverts back to its original size.
Snapping Pieces Together
For starters, to make it easier to connect the pieces of interiors, you can change your snap-to grid increment... In the editor main window, under File, select preferences. In the window this opens, you will see a number of fields you can change. Movement Speeds allows you to adjust how slowly or quickly things move when you drag them with your mouse. If you change these, do it by small increments, in general, because it's a fine tune control and fairly sensitive on most systems.
Grid Snap Is the increment with which an object will move on the X, Y, or Z plane when you drag it. It defaults to 64, I believe, and as long as you remain with multiples of 2 or 4, etc, you shouldn't have a terrible time lining things up sufficiently with a little practice. Simply use a large number to move things around quickly and a smaller number for fine tuning once you have your pieces "roughed-in". You'll almost always want "Use Grid" turned on when you're first getting the hang of things, by the way...
When assembling interiors, I recommend setting Grid Snap to 128 -- this is about half the width of a typical interior piece, and a quarter of that for large (e.g. Daedric) room sections. Cave sections are also multiples of 128, so this setting will slot them together nicely.
Additionally, you can save yourself a lot of trouble by using a Northmarker object to indicate the direction of "true north" in an interior cell whose entrance is not a cardinal direction. While not truly necessary, it does add to immersiveness.
Note: Pieces do not actually "snap" together as legos do, but very close to that. They should line up with a vaguely visible seam, but no gaps when they are together properly. (I'll provide an example near the end of this post.) It can be okay to overlap pieces at times (and is pretty much necessary in some cases, like adding rocks to dungeons without edges showing...just look in the example dungeon I list at the end) but it is good form and practice to try and keep the overlap of pathway or "main level" pieces to a minimum for best results.
Angle snap is the degree an object will rotate by when you are dragging to rotate. Turn it on to keep a sense of order to your object rotation, if you wish. I believe it defaults to 45 degrees, but you'll likely have to lower that quite a bit or turn it off entirely to get the proper angle you need to line certain pieces up properly.