mainsheet diameter

[BAtoo]

You will be fine on 10mm polyester. Jusy done the same on my 38s5 & going to use 12mm max but might drop to 10mm Spectra. Gone for Harken Carbo Blocks which look tiny compared with the old Goiot ones but have a high breaking strain.

[stevebirch2002]

10mm should be more than adequate on your boat. The Albin Vega (27 feet) uses 10mm for most of the halyards and sheets without any problems.