The mainsheet tackle sheaves on my C22 are sized for max 3/8" line. I could easily use 5/16, but the logic is that the larger diameter is easier on the hands for frequently handled lines. That doesn't mean go 1/2", that is huge and gets heavy on a small boat and I highly doubt your sheaves are sized for that.
Almost all of my running rigging is N.E. Ropes Sta-Set, just simple polyester cover over polyester core. For a cruiser that doesn't race that is all you really need. I check the bargin bins in the rigging section of local chandleries, they sell end spool lengths or other 'off' cuts of expensive dyneema core lines at 50% off so if it is a length and size I think I can use I have bought them. Dyneema line is kinda overkill on a small boat but it has its advantages, low stretch is nice on the halyards if you winch them up tight... I use 5/16 or 8 mm there. A low stretch Dyneema line is also nice on the Jib sheet, but that would get really expensive since I use 3/8" there (for the hands), but could also use 5/16 just as easily. Much of it is personal preference or driven by functional demand, but for our size boats you will pretty much see 1/4" for small stuff, then 5/16", and then up size to 3/8" for ease of handling but only if sheave sizes support it.
