CalTopo has become my go-to source for planning hiking routes. You can figure out mileage, check elevation gains, print maps, and even find trail heads (with the Google Maps layer). All free. There's a pay version that has some more useful features.
Like all FS and USGS maps there's a chance they're out of date, but being that it's a website CalTopo is always up to date with the latest maps (which still could be wrong, but it's the best you can get).
I typically print out sections of trail at home on plain old 8.5x11 office paper with a larger scale than 1:20,000 so they're easier to read. If a map is more critical (such as in a more abandoned wilderness area rather than a well used trail) I'll go with 1:20k or 1:24k, but printed at FedEx on 11x17. When I was organizing hiking groups I'd use the 11x17 as well and give everyone a copy.
Isn't the future great? There was a time not too long ago when this was impossible, and you had to buy maps that might have been out of date and showed way more than what you needed.