you need about 1" square for ever gallon (to be safe but mite need less or more), the width of your screen is irrellivent, but smaller widths need less GPH flow rate, for example a 1" width requires a 35gph but a 20" wide screen requires 700 (these numbers I pulled off a thread about them, I have never tested these). But over all, the taller the screen the thinner you can make it, therefor less gph, but more lights will be needed to reach the whole thing.
Best way to light them IMO is to use compact flouresents, in a cone reflector lamp to maxamize efficiency, also place them as close as possible, like only a few inches away. The screen type most ppl use is a "plastic canvas" which is normally used for embroidery and is sold at craft stores and some fabric stores.
http://en.wikipedia.org/wiki/File:Plastic-canvas.jpg Also its a good idea to rough it up a bit with some sandpaper. Here is a link to a massive tread on them, has an almost unlimited number of design patterns.
http://www.fishlore.com/fishforum/d...a-powerful-nitrate-phosphate-remover-diy.html
Quick list of required flow rates, also anything larger then 20" just add the the flow rates together like, for 40" you would need around 14000gph.
1" 35
2" 70
3" 105
4" 140
5" 175
6" 210
7" 245
8" 280
9" 315
10" 350
11" 385
12" 420
13" 455
14" 490
15" 525
16" 560
17" 595
18" 630
19" 665
20" 700