What is in the army how many figs and what kind of basing do the figs have.
It is best to do the figs and the display at the same time so they match.
You will need some supplies some thin plywood, some Masonite ( hardbord ) the same thickness as the figs bases get this at home improvement stores, and some cork you can get this in an office supply or Walmart.
The best size is relevant to the size and amount of figs in the group you don't want them bunched up it looks cluttered and you don't want too much space it looks like you forgot something.
But better too much than not enough you can fill in with terrain to make it look like what ever you want.
Determine the size of the board by grouping the squads and vehicles so they make a pleasing display. Make the board 2" bigger than this on all sides copy the size on to the hard board and draw around the bases these will become holes to hold the bases drill out the holes with hole saws and bits.
Glue the plywood to the the hardboard take to a frame shop or build your own frame and draw a line on the inside of the frame where it meets the hardboard this line is for the scenery don't go outside the line like coloring.
Now the hard part figuring out what to do for the scenery is all you the cork makes good rocks and concrete I also recommend drywall filler and inso foam post picks we want to see what you can do.