In reply to anomm: I use 5mm cord for my Prusiks and never had any real problems. They are very grippy on old 11mm ropes and when ascending a static, but having not tried other diameters I can't say if they are worse, but they are certainly OK.
In reply to anomm: I am equally happy using both 5mm and 6mm.
I would NOT use 4mm which I have seen others using. Equally, for recreational use 7mm is probably not ideal, although it is commonly used in MR and industrial rope access applications (which may explain the 'Beal recommendation').
In the last few years I've mainly used standard Mammut 5mm but that is because I acquired an entire drum of it. It works fine, however I still have an older and 'softer' 6mm prusik loop of unknown manufacture which arguably handles better. Some cord handles better than others, but I couldn't say which.
If you are going to be heavily using your prusik loop(s), such as a back-up when abseiling, I would buy 6mm. However if the loops will hopefully spend the entire season on the back of your harness then there would be little reason not to buy standard 5mm.