I get this question all the time and it depends on a number of factors. A few factors to consider include length of run, desired flow rate through the collectors, size of pump, cost of pipe and type of system (open loop, drainback or glycol).
From a cost and efficiency perspective you want to use the smallest diameter possible to accomplish the job. Because of the special requirements of a drainback system the minimum pipe size was discussed a few weeks back so I won’t rehash that. On glycol systems you have the chance to go with a smaller line set. Two of the requirements that you are working with for pipe sizing are; 1) keeping the velocity high enough that you keep air entrained in the heat transfer fluid to reduce the chance of the system getting air locked and, 2) keeping the flow low enough that you don’t suffer from pipe erosion by having too high a flow rate. To accomplish both of these requirements many sources recommend that you have a flow velocity between 2 and 4 feet per second. The following chart gives you the minimum and maximum flow rates that you should use for Type L copper of various diameters;
The column at the right gives you the maximum collector area that a given pipe diameter can support (from a flow rate perspective). This chart represents the maximum amount of flow for a given pipe diameter. Since higher flow rates lead to better heat exchanger and collector performance you may want to consider stepping up to the next larger pipe size where practical.