Well, the 10" frame probably won't accommodate the 12x12 bed, just as the 8" frame which I have won't accommodate the 10x10 bed... not enough width between the two Z axis pillars.

You might consider what I'm going to do, mentioned above: swap out JUST the Y axis pieces to accommodate the 8x12 bed. No wooden frame modifications are required. I don't usually need anything wider than 8" on at least a single axis, so extending the Y only out by about 4" is perfect for my needs.

If you're considering building a LARGER one, like a 12x12 (or bigger), perhaps go for the 8" now to get used to the process, then upgrade it yourself with parts from Open Build Parts Store (http://openbuildspartstore.com/). Most of the components will be the same (steppers, electronics, etc.) but you can bust out of the 8" confines by adding your own frames out of the V track aluminum.

There's also other bed-size options, like this 24x24 beast (http://store.quintessentialuniversal...id_product=113), but it will require a DC-to-AC110V relay rather than the usual DC-DC relays that most use.