I have seen 2 in NH. One was down in Allenstown and the second one was in Sandwich notch on the Black Mountain Trail. Both rattled and hissed and were about 3 feet long. Long enough to make me back away.
But nothing will beat the time I was in Montana on the Gallatin River fishing when I was climbing up over some rocks and got on top of a huge flat rock only to find about a dozen rattlesnakes coiled up in the sunlight. I nearly jumped in! They never moved but I sure did!