The tea bags don't seal shut -- not on their own, at least. I've had some success using a twist-tie, although a ribbon would probably have the same effect without being ugly. =P The twist-tie did reduce the bag's volume, but there was still plenty of room for two teaspoons' worth of leaves to unfurl and whatnot, and the tea came out nicely. (The bag tore when I tried to do a second infusion, though... but the metal end of the twist-tie was probably to blame.)
The bag is pretty wide, but I haven't had much luck flopping it over the edge of my cup. I've only tried it that way once, and it wasn't a great experience: The leaf-laden end of the bag wanted to pull the open end into the mug. I had to hold onto the open end of the bag to keep it from slipping into the water.
A rubber band stretched around the lip of the cup might work well -- it'd keep the bag in place and prevent the leaves from falling out -- but I've never tried it. A teapot with a lid would also probably work well, but I haven't tried that either. I don't suppose either of those is useful if the bags are gifts, though.
The bag itself is about 3"x5", with another 2" of paper at the opening. (It looks like this 2" was designed to fold down into the bag, but this doesn't really work.) Although I'm not a huge fan of them myself, the price can't be beat: $3 for 100 bags is almost too good to pass up, especially compared to the price of the RealiTea bags. =P If you're planning to place an order soon, I'd recommend tacking them on, even if only to experiment with.