The standard is text for 32 pages. That might mean one line per page, or more. 500-600 words is a good number to aim for. When it gets closer to 1,000, editors and agents may shy away.
I remember reading some Westerns in high school and, if I recall correctly, they weren’t terribly long. There wasn’t a whole about this on agent and editor sites, but from what I found, these can be anywhere from 50K to 80K. 60,000 is a solid number to aim for.
Memoir is the same as a novel and that means you’re aiming for 80,000-89,999. However, keep in mind when we talked about how people don’t know how to edit their work. This is specially true in memoir, I’ve found, because people tend to write everything about their life—because it all really happened.
Coming in a bit low (70-79K) is not a terrible thing, as it shows you know how to focus on the most interesting parts of your life and avoid a Bill-Clinton-esque tome-length book. At the same time, you may want to consider the high end of memoir at 99,999. Again, it’s a mental thing seeing a six-figure length memoir.
You have agents like Nathan Bransford (now formerly an agent) and Kristin Nelson who say that you shouldn’t think about word count, but rather you should think about pacing and telling the best story possible—and don’t worry about the length. Yes, they’re right, but the fact is: Not every agent feels that way and is willing to give a 129,000-word novel a shot. Agents have so many queries that they are looking for reasons to say no. They are looking for mistakes, chinks in the armor, to cut their query stack down by one. And if you adopt the mentality that your book has to be long, then you are giving them ammunition to reject you. Take your chances and hope that excellent writing will see your baby through no matter (and I hope it does indeed break through).
But I believe that we cannot count on being the exception; we must count on being the rule. That’s the best way to give yourself your best shot at succeeding.