I started RECENTLY with the Dovo Ebony Stainless 5/8 full hollow, in part because I wanted to start with a stainless blade until I decided whether I liked it or not. I got the set from SRD with a strop and Lynn's DVD, which was VERY helpful. Now I've got three more vintage straight razors with a short-term goal to have a SEVEN-DAY rotation, and they are ALL different in size, grind and metal. Dovo makes a GREAT razor (watch it on "How It's Made" 2010) - can't really go wrong. As I am still learning, I still use my Atra Plus on 2 weekdays when I only have 15 minutes to shave between working out and getting back to work on time, or when I totally screw up and my face needs to heal a bit! I'm considering a few 6/8" full hollows and half hollows now, but hey, that's all part of the fun.
(Now I found out TODAY that my wife's great-grandmother was a barber, and her Dad has her old straight razors! Can't wait to see them, now that I know a little something about the manufacturers, history and worth now.)
In general, however, 5/8 are at a decent price point and a versatile size, while the larger ones start getting more expensive. Even a 4/8 razor is a lot of blade to get used to (and be careful with) for somebody that's always used cartridges.