The Michigan permanent lifetime (non-expiring) trailer license plate fee starts at $75 and there's 3 catagories for unloaded (empty and dry as built) title weight stated from the manufacturer. 3 weight catagories with a corresponding lifetime fee. Length has nothing to do with it. Even if you had a 50 foot trailer or 5th wheel or utility or boat trailer etc, even if weighing over 20,000 lbs, the permanent plate max is $300 for a lifetime plate good for as long as you own the trailer.
We have 6 trailer with permanent plates presently and a couple years ago had 8. It has saved us many hundreds to thousands of dollars so far! 11 years of NO renewal fees times the number of trailers adds up to BIG bucks!