Hi Claudia, with so many questions I'd have to recommend that you contact a professional with the EXACT same questions. However, if this is something that you REALLY want to undertake yourself, then here's a few answers. Screen comes in 2' and 4' widths, the material (metal or synthetic) is basically a mute point, and simply put the width/height is relatively easy to figure out as far as which one will withstand the most wind, etc. Here's what I did years ago, I bought the 4' screen, made vertical posts every 2' and horizontal cross-members waist high. The screen was stapled to "EACH" horizontal and vertical member every couple of inches, then covered with a thin 2X which was nailed/tacked into place on the exterior of each member. By utilizing the 4' wide screen, but attaching every 2' created an sounder structure from the wind. Think of it as kind of like doubling-up on the strength, instead of 2 2' sections per vertical the stress is distributed across the 4' width (sorry, I can't really express this adequately in type). Anyway, that's what I did back in 1983, and everything is still intact.
Finally, one thing to check out is in *** ****'s FAQ section, they list a company that specializes in screened in porches. Just click on FAQ's to the left of this post. Hope this helps, and good luck! Wick