I'm running a local server via MCGalaxy.  I want a couple layers of grass in a large map; I tried  /os map resize 256 256 256, but I don't need grass that deep  and it just expands the map with empty space (no dirt/grass)

Look into /help newlvl and /help os map

I wrote a page about creating levels: https://github.com/ClassiCube/MCGalaxy/wiki/Creating-levels

I reread that,  didn't really help.  I ended up creating a new map 1024 16 1024 and using /resizelvl 1024 128 1024