Drake, the reason 1/2" tubing is used is because it is the closest size to the 16mm tubing used Europe. Full stop. When I started radiant, all that was available was metric tubing (16mm OD) and when Wirsbo first started making it here they needed it to fit some standard fittings and meet an ASTM standard so the 1/2" was used.
The 1/2" tube gives the best heat transfer with usability (remember that you have to pick up and move many rolls of tubing) and cost. It is a good balance. Where 1/2" tubing may cost $.5/ft (for example), a 3/4" tube will be close to $1/ft. There is no way the 3/4" offers twice the value and usability for the average residential build. I have used it on an aircraft hanger and for supply lines to manifolds.
The reason for the spacing is simple....the closer you can make it, the lower the water temp to get the same floor surface temp and transfer and therfore the more efficient the heating system will be. Simple.
It is no issue to make a 8" dia U bend, and there is no reason to try to get better than that even if you are having a 6" spacing. Don't sweat it, it is not that important. What is important is linear ft of tubing/ sq ft of space.
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