I can tell you based on my T-dozer base, which is huge, the length is 39" and the width 24". It's made out of 3/4" plywood.
I'm going to change over to a V-Slot base which will be smaller, although the gantry rail will be the same length and will cantilever over the base.
I will make the gantry rail 1000 mm which will sit on top of the v-slot base 380mm length by 600mm width (roughly 15" by 24" with a 39" gantry rail). You can make the base bigger but there's really no reason to do so given the weight of the printer is more than enough to cantilever the gantry rail over the printer base.
Here's an example from OpenDTG's store:
http://www.opendtg.com/shop/index.php?r ... duct_id=72
Hope that helps!