I have an 18" machine. That's the distance from the needle to the edge of the harp. When it's on the frame, without a quilt, the machine can only travel about 15", so you loose about 3" to the frame. On a queen, I can barely quilt the last row of 12" blocks, in a single go. So you would be able to quilt 12" blocks on the first row, but probably not all the blocks on the quilt. You need to ask yourself, however, if you really need to quilt a 12" block all at once. Can you quilt the top half (6") then the bottom? What size blocks do you commonly make?
There are lots of folks out there that have 9 or 11" machines on a frame and do lovely work. Depends if you want to work around something or not.