Hmmm....
oOo Oo
say you have 3 small and 2 large
Divide the tank into thirds, then make lines down the two lines on the right and left.
Try placing one large, just off center to the left line. Place one small stone on each side. Make sure the rocks are leaning the same way. Then go over to the right, put in a large stone on the right line, and put the stone to the right of that.