X-rays are all I had, and then the later mention that there was discussion in the surgery room with the rep of going down a size next time. Do they not measure and anticipate? Damn!

I never had CT scans on my knees before surgery, both were done successfully a couple of years apart with only lots of X-rays.
I did have CT and MRI scans of my shoulder when the same surgeon did a reverse shoulder replacement. The scans were to determine whether my rotator cuff muscles were usable for a traditional TSR (were not) and a later special scan to calibrate a custom prosthesis.

If there is no muscle or ligament problem with your knee before surgery (like an accident) that needs to be taken into account, then the X-rays should be sufficient for showing bone condition and cartilage loss. CT's and MRI's are expensive and are primarily used to scan soft tissue when that may be an issue, but in a "normal" TKR it is unlikely to provide any more useful information than X-rays .