For adult female rats and young rats 1/2 inch bar spacing is best and for adult male rats don't go any larger then 1 inch. You will find most cages have 1/2 bar spacing. Just a warning though, DON'T use tanks for rats, or even tank toppers!
Some cages like the ferret nation, have a little larger of bar spacing. I think it's close to 3/4 inch or something like that. Many "starter" cages for rats have 1/2 inch spacing. But beware, those are STARTER cages, and shouldn't be used for an adult rat, much less two or more (which they need to be in groups of). I use the starter cages for babies, or for a safe place to put the rats in when I'm cleaning their cage. Remember when it comes to space, bigger is better, since rats can't have too much room (as long as they have plenty of hiding places so they feel safe).