Like I said in the other thread, brakes and however your steering knuckles (added this part) are designed come into play with rim size. You do need some minimum clearances to clear all the mount points attached to the hub. You could negate this some what using wheel spacers but you'd have to run an ungodly large spacer to clear the calipers.
Now if you inboarded your brakes like on this '69 Jag E-Type, you could probably run that 5" rim of yours
But you can basically run as small of a rim as you can clear.