My first colon tube was a 32 Fr, 30". It's a great sensation, but I found I prefer a larger tube. My current favorite is a 40 Fr 30" with a closed end. I have a 48 Fr, 50" tube, but I rarely use it since the length is a bit much to control, especially once my hands get slippery from the lube. I'm considering cutting about 20" off of it and that might improve the control.
I tend to agree with that 30" is about enough. I know that there are folks who prefer longer tubes, but everyone's body is different. 30" is comfortable for me when inserted all the way. Well, all the way except for the inch or so I'm still hanging on to .
I think if you start with a 32 Fr 30" you can't go wrong. The closed end is better, in my opinion, since it's not susceptible to clogging up if your clean out enemas weren't entirely successful.
Just remember to take it slowly and gently. Two inches ahead and one inch back, along with a gentle twisting motion generally works very well. If it's done right, it feels great. So if you feel pain, you're doing it wrong.
To me, the colon tube is the ultimate type of enema.
Enjoy,
ldb