ZDooM is set up for skies that are 256*200 (i.e., 256 pixels wide, 200 high). However, a somewhat taller sky would be better for when you have a high height differential between outdoor sectors. Having said that, there are several excellent "tall" sky textures (i.e., 256*240) that are downloadable. Stian Skjondal created over 20 such skies, many of which are stunning.
In ZDoom, there's a setting for "stretch short skies". Since stretching skies really makes an environment look like shit, and in a big hurry, I was wondering what the normal size for ZDoom skies are, in pixels.
The DooM engine and ports are not set up to have 2-sided sidedefs tile vertically. One way to create the illusion of tiling, is to have a similar linedef parallel and very close to the first lindef, but vertically offset by 128 (remember to lower unpeg it too). This way it will appear that there is a single, tiled sidedef. Depending on what you're using the linedefs for, you can have a third linedef consisting of one of the STEP textures, again in parallel to the others and very close, offset by 120. The STEP texture will create the appearance of a bar across the "tiled" sidedefs and will conceal the slight gap between the first 2 linedefs.
Also, I'm having a hard time getting a two-sided linedef's texture to stretch vertically farther than 128 pixels. If anyone here knows what I mean, then e-mail, otherwise, I'll clarify.