As always...it depends! What size jib do you have? A genny may need shorter lines than a blade. And how is your main rigged?
Personally I use 1/2 on the jib sheets, 40' each. In theory you could go smaller diameter, but smaller lines didn't work on my (original) self-tailing winches (they tend to slip). And I like the feel of 1/2". And the overkill diameter means I don't worry about stretch so much.
For the mainsheet I have upgraded the blocks to give me a 6:1 purchase with two control lines, so I need a lot more line than usual; I used 65' of 1/2", which is probably 5-10' more than I actually need. I opted for Regatta Braid which is a bit softer on the hands, and has worked out well for me so far. You may want to look at that type of line.
Not clear about stretch limits - modern lines really don't stretch much, and a 1/2" is way overkill for the forces you are likely to be facing...double braided lines, of course.....I normally use the sta-set for sheets, but have occasionally used Endura. I am generally sailing 15-25 knots, and have not yet had any problems. 1/2" Sta-Set has 9000lb breaking strength and about 2% stretch, as I recall....Endura has effectively no stretch, but is 2-3x the price.
Hope this helps.