i think it's the size of the entire block that's important, not the number of pieces that it takes to make the block. in my opinion, a king bed looks good with a block (repeat) size of at least 8", unless it's a postage stamp or something very similar.
if the pattern is made entirely of borders, as in an outward going medallion, those pieces should be a little larger also, or the pieces get lost in the sauce. there's no place for your eye to rest.