I was just linking the German stuff because I didn't know what's common where you live. I really like Fischer and have used them exclusively now (and they are easy to come by, me being in Germany).

/u/calcium has linked the chart already, but for gyspum board you're basically limited by the material. I'd use a 6x30 or 6x50 for a single sheet. I prefer more holes to bigger ones in Gypsum, because you need to spread the load.

If you have a solid wall, even a shitty porous one (had old, loose, sandstone once) you want a long dowel so the load is as far INSIDE the material as possible. That way, you won't blow out the surface. Sometimes you have to lock the dowel in position, so it doesn't rotate before it starts expanding.