There's no "right" size needle for any yarn. It depends on the gauge you're trying to get and, as Jan says, on the fabric you want.
I think I'd start with around a 10-1/2. If you get too many stitches per inches for the gauge, or if the fabric feels too stiff, go up to an 11, then a 13.
If you get too few stitches for the gauge, or the fabric feels too loose, go down to a 10, then a 9.
Your mileage may vary.
I also don't understand what you mean by "ladders" on garter stitch.
Explain a little more, please?
BaaBaa on Ravelry