Hi guys just a quick one ,I need to replace my main and jib sheets the current ones are 20 mm and I feel they may be a tad too big for a thirty ft. yacht. Any recommedations from you experienced gents out there would be appreciated thanks.

Yes .

Talking about a "28feet", 8.7m yacht, 3.7tons.
The 12mm on the head sail was too heawy in light airs, too old to boot, replaced with 10mm double braid. Happy as Larry. Running a continuous sheet from clew to clew makes a good sense too, to clean up the snake pit in the cockpit.
The main is 12mm also double braid.
If squeemish about double braid, go for spectra, you could lift your boat with a 10mm.

On the Adams 10, l race on, the 12mm genoa sheet is way too cumbersome.

On the other one, an Adams 10.6, twin 8mm on the main and 10mm on the genoa, both double braid. The genoa sheet is continuous from clew to clew.
All halyards are 8mm spectra on a brand new mast.

If you have self tailing winches just make sure the "New" size works!