For all the jobs I applied for, my cover letter was one page. This included R1s, R2s, and SLACs (no community colleges). I would say it is a bit more than just a formality; you should demonstrate that you know what the job is, address specifics from the ad (can you teach the courses they mention? can you supervise undergraduate research? are you interested in publishing/presenting with students?). For teaching-oriented SLACs, my letter had the following paragraphs:
1) The job I'm applying for (name the school/position), my credentials (PhD in hand or expected graduation month/year), and a general statement of why I'm a good fit (e.g., given my enthusiasm for teaching at a liberal arts college and my past success in teaching, I feel I am a good fit for the job).
2) Specifics about teaching — the courses I have taught, the courses I'm interested in teaching — making sure to emphasize the courses mentioned in the job ad as well as courses specifically from their catalog (using correct titles, different schools have different course titles).
3) If there are research expectations, I'd briefly describe my research interests, and how excited I am to get students involved in that research. Emphasize the importance of student research experience as a key aspect to a quality education in psychology.
4) Sometimes, if the job was in an ideal location, I'd have a very short paragraph about why I'm excited to live in city X. But I only did this if I had a reason for being interested in that location.
5) Concluding paragraph mentioning other materials included (teaching statement, vita, etc.), note that my references are sending letters, and mentioning any upcoming conferences I'll be attending in case they would like to have a meeting/interview at that conference.
6) Sign the letter — even for electronic submissions, scan your signature and put it in your letter.
It sounds like a lot but you can fit it in one page. You might have to reduce your margins and/or font size for some letters, though. I know others that had longer cover letters, but my mentors all felt that one page was the perfect length — any more and it gets burdensome for the search committee, but any less and you might send the message that you're not interested or didn't research the position. And, I did end up with a TT position at a 3/3 (2 preps) with research expectations (I got a decent start-up and my own lab), during my first time on the market, so I presume I did something right!
Best of luck in your search!