Hi Jeff
Not sure if you’ve checked out the specs for the Journeyman (Journeyman Spec Sheet) but I’d give yourself an allowance of at least a few inches around the dimensions that Onefinity specify.
I believe that Onefinity recommends a table size of 49" x 72". (Although a plywood sheet is sold as 48" x 96" you may find that it is slightly larger than those dimensions)
As for the base material for the table something like jut one sheet of cabinet grade plywood would be ideal in my opinion together with a sheet of 3/4" MDF for your actual spoilboard. Just my 0.02¢ worth.
Cheers … Dave