What size needle should I use for two strands of cascade ecological wool held together?

[knitknitter]

I am trying to make a garter stitch jacket and want to make sure that the garter stitch looks neat and does not have those ladders that I see on some people's garter stitch.

[Jan in CA]

Ladders on garter stitch? Hmm... not sure I get that, but.... the yarn is already bulky so you'll have to use quite large needles. The best way to find out what size to use and if you like the fabric is to make a swatch. That way you can do what works best for you.

[knitasha]

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?