Homebrew has to be installed, you can't run it from a USB stick.
The only PS3 without a HDD is the 12gb superslim. If you have one of those then CFW isn't an option anyway, all indications are that you would have to redo the exploit every time you turn it on. You'd be able to install homebrew on the 12gb, but you probably would want a hdd eventually.
I don't bother with very large hard drives in PS3's anymore, as it's easier to stream iso's from a PC/NAS. If you play a lot of games then updates and saves take space though, minimum drive I'd go for is 160gb. Some people however decide to fit 1.5tb drives (I think this is still the largest you can use)