You can make the slot the size you want it to be. I believe anywhere from 1-3 GBs. Obviously, if you make them 3 GB's, you won't be able to use 4 slots. Each slot will be larger than what you make them as there is additional overhead for the formatting and such. I believe Hashcode said they require an addition .8 GB's of space. So a 1GB slot would actually require 1.8 GB's of free space. I could be wrong on the additional space required as it's been a while since I read that but I believe that's the correct amount. So, if you use 2 GB's of space per slot, you probably can only use 3 slots instead of 4.
I just use 1 GB for each one. It's plenty of space for the apps I have and that's all that's really necessary. I haven't had a problem with that size yet but I suppose it depends on how many apps you have so it could be different for you.
As for your stock slot, it will work. Just activate another slot, then once you want to go back to stock, activate it and boot to it. However, don't flash anything to the stock slot. Flashing to the stock slot doesn't work very well for some reason and you'll more than likely just end up losing your stock slot. Leave it alone and play with the other slots.