There are 2 sizes.
FIT: 1 3/16" inner diameter and 1 15/32" +/- 1/32" outer diameter (30.16mm / 36.95mm)
FIT L: 1 7/16" inner diameter and 1 21/32" +/- 1/32" outer diameter (36.51mm / 42.16mm)
Measure the widest part of the flaccid glans. The glans should fit inside comfortably.
What is more important than the glans fitting inside of the bell is:
The foreskin needs to make its way out around the bell and up the side.
The responsible answer to taping the meatus closed would be YES. The real-life answer would be "its a personal preference". I've never had an issue with air traveling up the urethra.
Feel free to ask more questions if you have any.
These pictures are of the FIT L.
The one on the left is a toilet paper roll tube.
It has the same outside diameter as the FIT L.
If your foreskin can extend around and up the side then the FIT L is for you. If you can not, order the FIT