It's an escape artist...

You may be onto something with it being too tight - it's going to change size no matter what happens, and probably needs a bit of room to be able to get back to filling out the tube. If it get's really short, why not just go with a "cap" style and leave it permanently short?

 

Think about it, there's no diameter of tube that you would be able to get into, that the size will prevent the tube from coming off. The penis is so compressible it's not the shaft that holds the device in place, they're not called "ball trap" devices for nothing.

An exception I have seen is having a ring behind the glans that is smaller than both the head and the rest of the tube. If the glans ring is sized so that you have to squeeze the head past it, then the glans will effectively 'hook on' to the ring and hold it in place. I haven't seen this configuration in off the shelf devices, a few custom manufacturers would do it for you.

It's the base ring / ring gap that is the most important measurement, if that's too large, then the whole device will be loose and allow you to move. You need to be sure what size of base ring and gap works for you before nailing down the other measurements like tube length and diameter, as when you change one it affects how the rest interacts. Even a very small difference in ring gap will change the angles at which a device can rotate, both up-down and around, and that determines what tube length and diameter would work best, but these latter measurements are the least important.

Another factor that can drastically change the comfort and security is the shape of the base ring.
Some people fit a circular ring best, others need a slightly oval base ring, with the longer axis either vertical or sideways. Just a few millimetres difference in diameters can make all the difference between comfort and reliability, and complete unsuitability. It's no surprise that most guys get through half a dozen devices before they find one that's really comfortable for long term wear.