Since I made a comment about this elsewhere online, and people found it really helpful. Someone had asked about length of cover letters.
My major determining factor has become how long the job description in the ad is. (Once you take out the necessary boiler plate and other more general stuff.)
If you have a description that runs the better part of a page, you’re going to need at least that much space to talk about it in your cover letter. Some things can be combined, of course, but on average, my cover letters were the length of the list of specifics about the job, plus a brief opening and closing paragraph.
This usually came out to about four or five paragraphs plus the intro and closing, depending on what the duties of the job were and how it made sense to group them. It was common for me to do a paragraph about reference tasks, one about instruction or training, one about technology, and one about supervision, for example. When applying for my current job, it made a lot of sense to take a paragraph to talk about my interest in accessibility, even though I didn’t have specific background in the field of blindness and visual impairment.
Opening paragraph was a basic “Hi, I am interested in This Job You Have Here.” and a sentence pr two about the kind of job or library or whatever made sense as a way to get into my main content “The position’s focus on reference is a particular attraction for me…” or whatever. Closing paragraph had any logistical notes (my last hunt, interviewing on Friday was much easier for me than other days), plus general “Thank you for your time and attention, I hope to hear from you soon.” or something similar.
If it’s a more general ad, with duties that are common in the field (and also obvious from the resume) then you obviously need a lot less space to say “Yes, I can do that thing.” though my letters usually still run about three-quarters of a page or about four moderate paragraphs.
Related: I have a longer post about my 2014-2015 job hunt, but I keep forgetting to check stats on files on my computer when I’m home. Mostly because I never did write up the summary at the time – moving on a tight time frame and then launching into a job where I had a lot of content learning to do will do that to a person – and it’s come up a couple of times recently. You should see it in the next couple of days.