It would depend, of course, on how big a head of romaine (or bunch of greens). The bags are on the small side (13-14" tall), and I bought them specifically to hold smaller things, like mushrooms or shallots. A bunch of kale, for example, would definitely not fit in...unless you didn't mind that the top of the bunch poked out the top.
I think they probably would be great for storing onions in your cold cellar over the winter. They are perforated for air flow which is why I really love them for produce. I would say they could hold like a 5-pound bag of onions you buy at the grocer. They really are great bags. Hope this was helpful.