The retainer must be small enough to be reasonably easy to install/remove, and it must be big enough to reliably anchor itself behind the inelastic ring of urethral tissue. If the retainer is too small, its rounded outer edge may be the only part that contacts the urethral wall. This would tend to push the surrounding tissue outward and possibly cause unintended stretching. The right size retainer, on the other hand, will have contact with the urethral wall on its flat part, so a pulling force on the spout will be resisted axially, without exerting undue radial forces.
To precisely figure out the size of your inner ring, you will have to probe it with a "urethral sound". Basically, this is a precisely sized rod that you insert into the tip of the penis to feel which size fits and which does not. "Sounding" is something that people also practice for the sheer feeling of it and not only for measuring purposes. To that effect, you can find sounding kits that feature multiple sound diameters, generally in steps of 1 millimeter. This is actually too coarse a stepping for our purposes here, because retainers are sized in units of 1/3 millimeters.
The unit used for retainers is called "French" (abbreviated "Fr"). It is used in the medical field for catheters and is convenient in that context because it also relates to the size of the incision needed to insert a catheter (you can read more about it in the Wikipedia article on the French catheter scale).
For measuring yourself, I recommend you order a "Sizing Sound/Dilator" from Tether Products. These are graduated directly in Fr units and are stepped smoothly along their length to make measurement easy and comfortable: you just insert the sound until you meet unyielding resistance. At that point, the last diameter inserted is your measurement.
Here are the two Sounds/dilators that are available from Tether Products. The metal part of each sound/dilator is about 6 1/2 inches long:
This 26 Fr diameter is a significant one: it corresponds to the smallest retainer available. Note also that the spout flange itself will require 25.5 Fr to get in. If your size is 27 Fr or above, you may want to use a retainer that is one Fr unit smaller than your measurement, to make insertion/removal easier. If your measurement is less than 26 fr, you will need to stretch your opening (the smooth steps in the sound/dilator will help here too) before you can use a TetherSpout.
When you eventually purchase your retainer, you will actually have to order a set composed of a round retainer and two oval ones. Here are two oval ones:
Each of the retainers in such a set can pass through the same opening, since they all have the same width. Personally, I don't use the oval ones because I find them more complicated to remove (you have to line them up so that their width and not their length is what passes through your urethral opening and that's not trivial).
Regarding spout sizes:
All the spouts have the same diameter (it matches the size of the hole in all the retainers). When you buy spouts, you can opt to get a set of 4 different lengths, which is slightly cheaper than if you were to buy all 4 separately. My advice is to just get two of the longest (24mm) spouts. For chastity purposes, the shorter ones are not that useful. If you do buy a set of 4, you'll probably end up wearing only the longest one and you'll use the next-to-longest only to help insertions/removals.
Remember to sanitize your sound(s) and the tip of your penis thoroughly before insertion (e.g.with antibacterial soap). Especially as you'll probably push the sound in further than any spout. It is also a good idea to urinate after you are done to flush out any possible impurities.
PS: I'm not sorry for the possibly misleading title. I like double-entendre. :-p
