I was in this position about a year ago. My advice is to get as big a shed as you can. 5x7 won't leave you much room to move the bikes around. My experience is that metal or plastic sheds potentially are more secure but are much harder to erect and make watertight and always have condensation problems. I'd go for a wooden shed if I had to do this again and put some sort of ground anchor in before I put the shed up to chain the bikes too.
My recollection is that there's a good choice on the net. I used
www.shedstore.co.uk. Prices were good and they delivered on time.
If you just need it for storing bikes (as opposed to working on them) I wouldn't bother with windows (more secure without). At 5x7 a pent roof's probably cheaper and easier to erect. If you go down the plastic or metal route allow for lining the roof (and maybe walls) with polystyrene insulation and for putting down a good base and frame to mount on.
Hope it goes well.