Talk to a printer. My local printer does 11x17 and 12x48 on light cardstock with no problem. It costs money, but it makes a great map. [And it generally costs less than the minis I use on it.] A 20x40 map fits on two 12x48s.

I tend to focus on generic maps I can re-use in many situations -- a forest trail, an open field, a seacoast, a canyon, etc.