Here is a very large sign cut out of 16 gauge steel by @DnKFab
It will boil down to the supports built into the structure of the sign. If you grabbed a piece of metal and lifted it, you will get a sense of home much it is flexing. Too much flex and that sign is going move/shake in the wind. If it is exposed to the wind and there is not enough relief for the wind, it will have huge forces trying to topple the sign.
If the sign is up against a wall, then 16 gauge would probably work. If you have big holes/openings that will subject all the weight on small bands of the metal then it might distort and bend the sign as you move it around the shop, transport, install. In that case, you might consider something thicker (or build some support structure on the back- bond it with some epoxy or something like JB Weld). Aluminum seems to have less flex than steel but it is a bugger once it gets a bend/kink.
Lots to consider.