most subs comes with spec sheet's that say exactly what size of box to put the in. If you need it before hand, like Knacko said, just e-mail them or look on their site.
btw, did you mean 2-12" Infinity's or 2-12" Type R's?
every sub is different. there isn't one set size enclosure for every size sub. one 12 may want .6 cu. ft., another 12 may want 2.6 cu. ft. like knacko and young james said, either look on crutchfied if the subs there, or go to the manaufacturers website or email them.