Every 0.33 millimeter (mm) of thickness /diameter, measured on outer walls contributes 1 FR.
that is, 1FR=0.33mm

What this means is that a 12 FR tube is 12*0.33 = 4.26 mm thick,
while a 16FR tube is 16*0.33 = 5.28 mm thick
...and so on

So, if you want to get an idea of the thickness of the tube, simply use this formula:

Thickness (mm) = Size in FR * 0.33

(Source: http://en.wikipedia.org/wiki/French_catheter_scale)

This is purely a personal choice.

Our 12FR Disposable Sterilized tubes are the most popular, which perhaps gives an indication that this size is preferred by many. This is also the standard size we supply FREE (1 tip) with our Enema Kits.

But, many people prefer thicker tubes (32/ 34 FR) because they are more rigid, do not kink easily and deliver the liquid faster.

On the other hand, there are others who prefer thinner tubes to insert inside their colon.

If you are not sure, perhaps the best way is to have a size somewhere in the middle (such as 20FR) and then experiment whether you would like ti thicker or thinner from thereon.