The 35 gallon tub is 16 inches wide at the bottom, and 22 inches at the top (since the water causes the plastic to bow.)
And it's 31 inches long. It would still be more comfortable than a 30 gallon tank!
And the 60 gallon tubs are even bigger. I wouldn't fill a 60 gallon tub all the way up with water, unless absolutely necessary, and maybe get some dowels or other type of wood to put under the outside lip, to help keep the plastic's integrity in line, to prevent breakage.