I used F-channel against my shed to hold the soffit, that way you can make the height whatever you want by using the wall to nail it into. I also used an F channel nailed to the fascia board, hanging down to hold the outside end of the soffit. I'm not sure what the max span can be without nailing the middle of the soffit, I think it was 16", but whatever it was, my shed was within the max. If you are outside of that, you need a nailing strip to nail the middle of the soffit to.
Then, I used vinyl fascia. On the top of the fascia board I installed drip edge under the shingle and a "sil" piece for the fascia to snap into. I cut the fascia to height, punched the snap holes into it and pushed it into the sil, snapping it in place. The bottom of the fascia wraps around and snaps into the F channel already there for a "nailless" solution. On the corners, I used sil vertically on each side for the end of the fascia to snap into, and caulked inbetween the 2 sill pieces.
I chose vinyl soffits and vinyl fascia because I could do it myself without expensive tools. The punch was $10, and the fine tooth blade I put on my circular saw backwards was $15 or so. I already had snips. Hammer, nails, and caulk are the other supplies needed, besides the obvious vinyl system parts.
See some illustrations here:
Fascia
http://www.variform.com/NapcoWebDocument.nsf/LookupInstruct/FasciaInstallation/$file/Fascia%20Installaiton.pdf
Soffit
http://www.variform.com/NapcoWebDocument.nsf/8c88fd58dc1710c186256b0a0055dec3/3436fb489368576a86256c83005b4924/$FILE/Soffit%20Installation.pdf
Hope that helps,
-Ed
