I like to use the blocks as my guidelines and use a size that corresponds to the blocks. If my 12" blocks have 3" segments, I can go with 1 1/2" for inner border and 3" or 4.5" for outer. (Or something like that.)
Other times, the amount of fabric I have left over determines the size of the border. If I don't have enough to cut 4.5 it will simply be smaller.
For smaller quilts in particular, the availability of backing fabric may determine how big I will make the top (and the border). Not sure there really is a rule. It's all visual impact and personal choice for me.