The 6' boards should work fine for a 250 pound rider, but it might bog a bit on the start. Why not give the long board a try, then try the shorter one.
After I blew my knees up I quit wakeboarding, and now I just wakeskate.. and if I can get my 180 pounds out on the end to hang 10 on a Cassette I am sure you'll be ripping on a 6' wakesurfer in no time...