I'd consider their mouth size to be a little wider than the distance between their eyes. If the widest part of the small frog is bigger than that you should be okay.
Edit: or to put it more clearly- the width of the head at the widest part is the same width as the mouth.

It's tempting to want to put them together, but if you think the little frog could get eaten I wouldn't do it. My frogs usually ignore each other, but after a while I saw the big one chomp down on the little one's head. Luckily the rest of the little frog didn't fit in his mouth!