How closely does the size of a chainring guard have to match with the size of the chainring you use it with? Or, put another way, what are the consequences of using a 39 or 40t ring with a guard intended for a 50t ring?

Reason I'm asking, I've got a double crank I wanted to get rid of, but thought of replacing the outer ring with a guard and running only the 39t inner ring for a fixie/singlespeed setup. I happened to come across a chainring guard for cheap, but it's said to be for a 50t ring, and I've got no experience to tell me how big a problem that might be.

Thanks!