Three feet high, or doorknob height should be fine for anybody, even those who have had colon surgery. That makes about 1 1/4 psi to the floor, or about 1 psi in the colon, since it is several inches above the floor. I'm NOT talking about the anus which would be two inches above the floor. The colon sits higher up inside your body, and 'm not going to get into three significant figure precision here because ten decimal accuracy doesn't make any difference, it just makes you look smarter to those who don't know anything about math and significant figure accuracy. I have posted elsewhere exactly what the pressures are.
For "normal people," five feet is perfectly fine, but that is as far as I would go--that is about 2 psi.
OK, I couldn't get a direct link to the exact post, only the thread, so here is what I said:
This is how it is: One pound per square inch pressure (1 psi) is generated by a column of water exactly 27.68 inches tall. That is very close to 1/2 psi for every 14". That means five feet or 60" gives a pressure of 2.16 psi. The colon can easily contain that pressure, but that is as high as I would go. Above that the pressure starts to get too high and it will feel uncomfortable. Unless you have a really strong sphincter, you will probably need a double balloon rectal catheter to help retain that if you plan on holding it very long. Bigmike is correct about the final pressure in your colon being determined by the final height of fluid in the tube after your bag is empty or you are completely full and stop taking in any more fluid, but the presence of blockages can increase the pressure, however, they still won't be more than what is dictated by the height of the bag.
The diameter of the tube does not affect the pressure. Pressure is defined purely by column height. If you have a very small diameter tube, it will flow slowly, if you use a 1/2"ID diameter tube, your guts will fill VERY FAST, especially if you have been cleaned out first. Have fun!