Eh Greebo, let me know what you think of my updates to avachanl running in the AvA for the next two weeks. /endshamelessplug
Hmmm... well, 10yr+ old regulations, and typical AH user now can download more than 10mb an hour....
How much of an impact in size are we talking, within reason (a couple/few custom textures/objects that is then stamped/copied many times over the map)? 10/20/50/100/200/500MBs?
Taking your post as a request for information: At the default texture size of 512x512 pixels, a full set of LAND textures would add 12 textures x 257 KB = 3 MB, and double that for water textures. The good news is that it would be unnecessary over kill. A better answer is to select a few key textures like beach, grass, two farmland and possibly the two forest textures, and then create 'high res' 1024x1024 textures at 1MB each. That's 6 textures x 1 MB = 6 MB in additional size. Oops! Okay, you probably won't get a way with adding 6 MB to your MA terrain size.
For insight why you'd want to increase the texture size; each ground texture covers 4 square miles and a single pixel covers 20.6 feet. Using a 1024 texture covers only 10 feet of ground per pixel, and that works much better for drawing things like trees and clumps of bushes. The terrain can look pretty spectacular with that size texture.
Custom objects are a different story. If you can use existing textures to cover the surfaces, then their size is negligible. My arched stone bridge adds 20KB, then it's used repeatedly without adding more to the terrain size.
Custom objects are not allowed in MA terrains because of quality concerns. Concerns like missing texture mapping making them see-through from one side. Or surfaces that drop through the terrain, which can also cause a player to drop through the terrain.