Sneak + Rightclick replaces blocks with whatever is in your offhand (aka put a diamond ore in off hand, and with staff of building out, holding sneak will not focus entirely on existing blocks. right click to replace)

Also what would would work this is expanding area/shrinking area.

Sneak + scroll while staff of building is in hand and you will change the area selection ie; 1x1,2x2,4x4,8x8,16x16, etc etc.