For an upwind / reaching sail - keep the sail luff length below the max top block to tack attachment point. Foot length as you please, as long as it fits the aftmost position of the block that will lead the sheet. You want to controll all edges.
For an off the wind
/ broad reaching sail - you can go for a longer luff.
Cloth weight - get a slightly heavier sail for the upwind sail, unless it is meant to be used exclusively in very light conditions. You can still use very light cloth though - if you can afford the more expensive, less stretchy materials.