jms,
At least I can assure you that the rectal probe used during your urodynamics test is relatively small and will not be uncomfortable.
Because of some complications caused by my pelvic nerve trauma my tech took quite a bit longer than usual during my urodynamics tests. It could have been a very embarrassing situation as I had the female tech "fondling" my penis for about an hour. However, she was totally professional and really defused a potentially stressful situation. Later, I was able to joke about it.
As I had an inflamed and slightly enlarged prostate, the only uncomfortable moments were when the catheters had to pass through the narrowed channel through the prostate.
It helps to read about the procedure to be better informed as to what will happen, but I would not worry about it (I admit that I did worry about it).
I found it to be a relief to get the results of the tests in order to better understand what was happening.
I hope your tests go as well as mine did.
--John