I give them either an 8"x10" or an 11"x14 guide print, but I allow for some interpretation on their part, sometimes what looks good at 8"x10" may be too dark or light or something at 20"x24". I've worked with one or two printers there for several years now, so they know how I like my prints to look.
Originally Posted by Toffle
I think a big part of using labs is developing a really good relationship so that you and are ultimately on the same page. It can take several times going back and forth, but I think it's worth the effort. I think just about any lab can pull your vision out of your negatives, but it can take some work to get there.