They recommend at least 2" of extra press all the way around. So, a 32"x42" press can safely press up to 30"x40".
An interesting aside is that they will manufacture any size heat platen you want. I asked what was the largest platen that would fit on the DK20S frame. They said you can do anything that equals 500 square inches or less. They gave an example of an 18" x 28" press. The price is $3,850 for a custom sized press. This compares to the $2,975 list price of the DK25SJ (20"x25" platen).
In my search for a larger press, I just picked up a used DK25SJ on ebay. Ultimately, I still would like a bigger press. It's just a very big jump from the DK25JS ($2,975) to the 32"x42" Maxi Press ($8,950).