Stehno is absolutely right. The cone size is not important. Since your current main speakers go to 35Hz you'll need a sub that can crossover below this level (unless you filter the signal going to the main speakers).
My main speakers go to around 35Hz and I'm crossing at 27Hz to avoid boomy mid-bass !
My suggestion would be to look for a used REL strata. It crosses very low (down to 25Hz), it's adequately powerful for all but the largest room, and if you don't like it or ever want to upgrade they hold their value and are easy to resell.
IMHO Better no sub than a cheap sub for 2ch music.
My main speakers go to around 35Hz and I'm crossing at 27Hz to avoid boomy mid-bass !
My suggestion would be to look for a used REL strata. It crosses very low (down to 25Hz), it's adequately powerful for all but the largest room, and if you don't like it or ever want to upgrade they hold their value and are easy to resell.
IMHO Better no sub than a cheap sub for 2ch music.