When screwing around with drywall, I use 1 5/8" for both 5/8" on ceiling and 1/2" on walls. Never had a problem, however I recently read somthing somewhere that recommended 1 1/4" screws because longer ones hold better and you have more screws popping through the paper when the rock and house moves and flexes around with changes in humidity and seasons, etc. Let's see what the others say. I also noticed someone named drywall guy posting below so maybe we can wait for this guy to provide his expertise as his name implies.