I've used em all... The one I like is the " Original ". I'ma thinkin it's about 11 inches wide by 8 inches tall by 1.5 thick. I like this one because he will easily carry a big pistol or revolver, which is what I got him for.
As I remember, there is one that is a bit larger volume wise, which would work great for carrying first aid n edc stuff. If ya are smaller across the chest, they make a " stubby " which is a bit shorter. It will easily hold a subcompact pistola. I've carried the Glock subcompacts ( 26, 33, 27 ) in him easily.
These bags work exactly as ya would think, are of a bombproof build quality. There is nothing bad to say about em, other than they are pricey. Price is the main drawback, especially now that others are copying the concept.
I would look at the 511 stuff too, but I haven't tried their chest bag. Hope this helps a bit.
leroy...