I have the MDL and it works great. The largest diameter is 2 1/2" or 7 3/4" circumference at base. You might play with some toys and find out how much you really think you can handle. I wish I could handle the XXL just to get the extra length but there is little to no way I can handle the 3.5" diameter. You might just settle on the Large which is reasonable and you get an extra inch in length. The base will hold a light suction when you have it flat on the toilet seat though it does not have an actual suction cup on the base. I do like the material it is made up which helps getting it inserted and feels good as I work my way to getting it fully inserted all the way. Also the fact it is tapered is a blessing to me since I do not believe I could get as much in me if it were the same diameter for the full length. Hope this helps.. It has been a great addition to my collection and I find myself riding it almost every night now.
Small: Height 12″ • Circ 6 1/4″
Medium: Height 16″ • Max Circ 8″
MDL: Height 20″ • Max Circ 7 3/4″ or 2.46" diameter
Large: Height 21″ • Max Circ 9″ or 2.86" diameter
XL: Height 20″ • Circ 10″ or 3.18" diameter
XXL: Height 24″ • Circ 11″ or 3.5" diameter