For a boat your size, 3/8" will be more than adequate for size. 5/16" even smaller would work for light air sails
and halyards. The real determining factor is what is comfortable in your hands. !/2" is nice but heavy and the tails make a big bundle. Used 7/16" for sheets
and 3/8" for halyards on my 35' boat. 7/16" is kind to my arthritic fingers and not as heavy or bulky as 1/2". For Halyards, wouldn't go larger than 3/8' and possibly smaller as the heavy work is typically done with a winch
and they aren't tinkered with much.
For lengths, the height of the mast
doubled plus 5' or a little more should be adequate. Have heard 1.5 times the boat length for sheets. A low stretch line like New England
StaSet X or VPC would be better for halyards.