There are 3 sizes.
FIT S: 7/8" inner diameter and 9/64" +/- 1/32" outer diameter (22.5mm / 29mm)
FIT R: 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.
Take a 1 inch wide piece of paper and mark from one end 70mm, 92mm, 112mm. Curl the paper together so that it lines up with the markings. This is the inside circumference of the different size FITS.
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.