One option, mentioned in
Soldiering by Rice Bull (an excellent first person account of soldier life, btw), is to get a third man to share your dog tent. That third shelter half can be buttoned to one end to close it off and also provides a little more length as well as a handy place to stow knapsacks or other gear. Three normal sized fellows can fit fairly comfortably (and warmly) in a dog tent. If you need a little more elbow room just shorten the poles or sticks on the ends the tent will of course be a little shorter but also wider as a result.
Rice also points out that three men is also an ideal size for a mess in that it allows for one man to fetch water, one to gather firewood, and one to do the cooking.