Thanks to all who contributed to the various threads about sizing. It took a while to settle on what I think are the right measurements. I did use the sizing rings. MatureMetal was very good and very patient about explaining all the options, and I feel pretty comfortable with the numbers I just sent in on my order. I chose the ring diameter where it was tricky wiggle the head through but comfortably snug around my shaft; and I chose the base ring where it was a tiny struggle to get everything through but didn't feel too tight, and where I could still slip a finger in. The only measurement that was surprising to me was the length. I have measured very consistently at 3-3.25". On a rare occasion, very rare, I have seen it at 2.75". But Mature Metal said that 3" was longer than ordinary. It's surprising because I am not big. I am a satisfactory length (average), and a very slender girth (between 1" and 1.25" diameter) when erect. But when flaccid, I guess I don't shrink as much as one would expect? So I took the advice of the "smallest length" of 2.75" and subtracted the 1/4" for my measurement. Now the waiting for 6-8 weeks and we'll see how I did.
Sounds like you have done your homework and everything should be ok. I do not think tube length will be a problem. I am normal size .....but a grower and when flacid fit very easily into a small cage.