Don't trust my post unless someone more knowledgeable says similar things because I have never made a boulder wall. lol

Dig a trench and fill it with gravel to keep them from sinking too much? Atleast I think that's the standard way of doing it for retaining walls, guess it would work for boulders as well? And putting larger ones at the bottom seems very logical and looks nice. Maybe dig the bottom layer in the soil relatively deep for stability. And just try to stack the stones so that they are as stable as possible with as little gaps as possible. Remember to have the wall leaning on the soil, probably quite a lot since it's boulders.