Dear Miss Snark,
I'm sending out my first full manuscript to an agent. It's very short (65k). Would it be appropriate to mention in the cover letter that many similar novels (most notably Animal Farm and Fahrenheit 451) are also short?
It shows you haven't a clue about the difference between front and back list. Both of those books may be classics but they were published before most editors and agents were born. Editors and agents are looking for things that can be sold TODAY (and no comment screeches about how dare I suggest Animal Farm couldn't be sold today.)
65K isn't very short, particularly if you're writing genre fiction. It's just short, but within acceptable limits.
And don't compare your book to others. If anything say readers who liked the novel Miss Snark's Guide to Etiquette will also like this, or this book will appeal to readers who liked Miss Snark's Guide to Etiquette.