The trim size for sheet music is often idiosyncratic--meaning that it is peculiar to the printer, and often to the piece itself. I think that there are some standard sizes that you run across a lot, but there are lots of non-standard sizes. Lots of material is printed in non-standard sizes--the paper sheet sizes may come in standard sizes, but you can trim pieces to any size you want--


One reason that you might cut a piece to a special size is because the end use requires it--music is often printed on as large a sheet as possible because it is read from a much greater distance than
a book, magazine, or newspaper. It also is often printed in sizes that are convenient to hold while performing.

Another reason that pieces are printed in special sizes is to create visual interest. Sheet music is "Self-Advertising", meaning that it is designed to capture the attention of the potential buyer, so this is a factor, too.

Also, because music is used repeatedly, and over a number of years, special kinds of paper are used --that are more durable,that are more readable, that don't yellow, etc. Unusual types of paper often come in idiosyncratic sheet sizes--

So there you have some good reasons why music comes in a lot of different sizes..I suddenly remember another, and that is that some publishers deliberately cut pieces a little bit smaller than others so that they will always be stacked on top of the pile--