Gary, if you gonna print it by yourself just for yourself (hang the poster in your own room), there should be no problems. Different matter, if you will go public with it.
On how big you can go, there is not really way to tell from just this picture you are showing us. Neg is much larger then usual medium size which usually ends at 6x9 and obviously was shot with large format camera. The edge of the negative is showing typical pattern for LF negative carrier, so there is a chance a good lens was used, possibly very good lens. So there is a real chance you can get 10x magnification (print size 90 cm x 120 cm ) with excellent results.