Bear in mind that, when you use them as they're designed, smocks are intended to be outer layer bad (but not necessarily wet) weather garments. That means they're designed to accommodate a boatload of layers underneath and that's why they look like dresses when you wear them without.
When I was in the forces I bought an NLDPM Arktis smock in Medium. I was a little heavier then than I am now and it worked, but in some situations I would have liked to have a Large.
Also, in some situations (at least in the Dutch forces) you'd wear your body armour underneath your smock and wear LBE over it. Don't know about SOP's in different countries.
But to be perfectly honest, in my opinion smocks are a bit of a relic, especially in summer.