I started out with a 10-gal mash tun, and that works great for almost any 5-gallon batch. You're pushing it on big beers though.
When I scaled back to 3-gal batches, I built a 5-gal mash tun. The 5-gal tun has worked great for 3-gal batches, even an Imperial Stout (although again, it was pushing it).
If I were you, I'd consider doing 3-gal batches and fermenting in a 5-gal carboy. Incidentally, all of the recipes in Brewing Classic Styles are actually formulated for 6 gallons, so it's pretty easy to cut one of those in half.