The larger the block, the easier to use, the smaller, the less space required when hard sheeted block to block.
The thicker the sheet, the easier on your hands and therefore less fatigue but bigger blocks. The thinner the sheet usually cheaper and smaller blocks required also cheaper.
On short constantly changing course thinner sheets are ok but marathons with fewer course changes thicker sheets means less fatigue.
If you use a ratchet block this can alleviate a lot of hand discomfort

I agree with those comments above I have found also that using dinghy sheet rope from a yacht chandlers makes a huge difference Dinghy sheet rope is made for the job very flexible so it bends around the blocks easily and reduces
drag and is also soft on the hands.
Another worthwhile thing to have is what has been called a "Crockett down haul"
on these forums Besides working well the down haul itself only uses small cord and blocks

Thanks gentlemen,
I do have an understanding of the physics involved and the comparative effort between smaller and larger sheaves and indeed rope comfort. What I don't have is any experience in sailing, other than being banned by the hire guy at Yeppoon many years ago after brother and I nosedived one of his catamarans numerous times in the first 15 minutes of the rental.

effectively I had no yardstick to determine the parameters of what would be barely adequate, what would be acceptable and what would be optimum. So far I have been unable to source anything in the way of blocks that isn't priced at dollars per millimetre of diameter and was hoping to get it close to right on the first go. Obviously if 12mm sheet rope is a minimum then buying blocks with a maximum capacity for 8 mm rope would be a cockup.

Mick

Chook2 said..
Great advise Hiko!!!

If I remember correctly It was TP1 that told me to wash the salt off my sheet rope and then chuck them in the washing machine with a good lot of fabric softener added, to get even more flex from the sailing rope.

Makes it smell like you are a fairy.......... But makes a huge difference to the ropes handling.

Makes it smell like you are a fairy
Taking your size into account that would be a confident call

Possibly a fairy ELEPHANT