Oh Patrick, I made the mistake of using 1/4" I know it's cheaper and comes in a longer roll and the coupling nut is the same size as the hole in the pressure cooker. Having said that I am really disapointed in mine. The preassure is to great in the cooker and I lose steam thru the thermometer hole. the copper is to thin to insert a thermometer to measure the steam temp. do 3/8" if you can.

if you use 1/4" tubing you'll wait forever for all the vapor to come out. I would go 3/8 or even 1/2, or have a very large tube coming off the cooker into a 'slobber box', then have a small diameter tube coming off that.

You can get more cooling over a shorter tun by using 1/4"

The pressure problem with 1/4" is real. The solution increases your through-put.
Use more than one out-bound chilling coil. Simply tap another hole in your pot next to the first one leaving about half an inch between them.
If the pressure is still high: install a third.
The higher surface area exposure you'll get from multiple smaller copper tubes will increase the speed of chilling the vapor

Get yourself a pipe tap at any hardware store and some real silver solder. the kind with no lead. you can silver solder with propane if you are patient. tap the kettle and install the SST compression fitting solder it in for good and then hook up your tubing. make it as long as you need.