I pull the AICPA templates and create my own templates based on them. (Looking forward to making new templates once they've been updated for the 2019 filing season
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Never had a complaint about the length of them. They usually end up being 8-10 pages long for a compliance engagement, which includes my letterhead, closing paragraph, signature, and closing logos.
Haven't had a complaint yet about length, burden, or confusing language. I tend to leave out any of the "optional" paragraphs, which I highlight yellow in my version of the template so I can evaluate on a case-by-case basis, if I have reasonable knowledge they're inapplicable to the particular client.
In a nutshell I'm less concerned with length and more concerned with protecting myself as far as engagement letters are concerned.
All my clients are reasonable and know they can ask me questions if something needs clarification.
The real fun in late December / early January will be figuring out the route I'm going to go with organizers...not that it will matter much.