Typically, the MTE, MT1, and MT2 tower you would need the small size. That is going to hold two boards or one inflatable tube (roughly 16 pounds). The actual bimini top canvas size for the small is going to be 48" W x 65" L.
The medium size, on the other hand, is going to hold three boards or 1 inflatable tube and the actual bimini top canvas size for the medium is going to be 60" W x 65" L. We recommend measuring your tower before ordering per the instructions to make sure you receive the correct size.