I have one of the larger mats, 36 x 60, and put it in the regular bathtub, yes it will bow for the time it's in there, but that doesn't matter because when you pull it out, it's nice and flat once again- they really are flexible mats so don't worry about it bowing in the tub while it's soaking for 15 mins! I did mine and it's great now, lying flat on the table. So go ahead an try your larger mat in the tub, it will work out just fine!
Kind Regards,
MaryAnna