When I took my first case, I used a retainer that I found in the back of a practice manual. It was 9 pages if I used 10-point Times New Roman. Even I did not understand it all. I soon whittled it down to a 3- or 4-page retainer that was a little better, but I am pretty sure most clients took my word for its contents, then stared at the pages for a few minutes.
My current retainers are all one page or less, 11-point Georgia, with plenty of white space for readability. Your retainer is a sign that you understand how to communicate, that you are straightforward, and that you have nothing to hide. There is no reason your retainer should be longer than one page.
Here is my generic flat-fee retainer agreement (pdf). It has all the essential elements, although I haven’t tried to anticipate every possible situation. I focus on the most-important ones. The description and the fee are the keys, and we always use detailed descriptions so that everybody knows just what we agree to do in exchange for the fee.
I’ve heard that a local personal injury firm uses a one-sentence retainer, something like “Client agrees to pay Law Firm one-third of the recovery, after subtracting costs.” For my own contingent-fee retainer, I go a bit further, in part because I think the extra length adds clarity, not confusion. But it is still only one page, and my clients have no difficulty understanding it
If you aren’t convinced, run your retainer through Flesh to test its readability according to the Flesch–Kincaid readability test. If it requires above a grade 12 reading level, call it a failure, unless your clients are 100% college grads.
(You can use my retainer for inspiration, but please respect my work and my copyright.)