My longest ruler is a 24" ruler. (I prefer the Omnigrid 6 1/2" by 24") I have several mats - all the green Olfa brand. I have the 12" x 18", 18" x 24", and 24" x 36" sizes. I leave the 24" x 36" set up on my desk at home and usually take the smaller two on trips with me. There is one that you can get up to 70" (see here for the Olfa brand:
http://www.olfa.com/MatsDetail.aspx?C=27&Id=87). When I have stuff longer than 24", I make about 20" cuts at a time and carefully move the fabric as necessary.
You would need table (or floor) space big enough to accommodate the big mat and/or ruler. I don't know off-hand of a ruler longer than the 24" though.