as mentioned above, it depends on volume. 1 10"? or 8 10's? i read on here for SPL they got the best results from a little box with a big port. so if that's the case then you'd wanna go on the higher end of sq inches for port. of course it's not limited to 12-16 sq inches tho. that's where testing and testing and testing comes into play.
but to answer your question:
8" - 12 sq inches
10" - 13 sq inches
12" - 14 sq inches
15" - 15 sq inches
18" - 16 sq inches