Your frame width can vary across a pretty wide range. Generally speaking, the further apart the tubes are, the more stable the cataraft will be.
The diameter of the tubes enters into the picture indirectly. The bigger the tube diameter, the longer the oars you will need to reach the water at a manageable angle. For 24" tubes, your frame will be so high above the water that you'll need 10' oars. For 22" tubes, 9.5' oars. For 18-20" tubes, 9' oars.
The length of the oars determines the oarlock spacing. The oarlocks should be spaced apart by roughly 2/3 the length of the oars (this varies by personal preference--I like mine a bit closer together than 2/3). In theory, the oarlocks or pins for your 9' oars should be 72" apart. Use that dimension as a starting place for your frame width.
The other key dimension in determining the width of your frame is the length of your ice chest(s). My Yeti is 42" long. So I wouldn't want a cataraft frame that has any less than 42" between the tubes.