They generally are the generic 82*5.5*1.1 variety, but there are other lengths available, e.g. 50mm, used on some battery powered planers and in CNC router tooling.
Largest I use is 82mm, smallest is 20mm, all the same 5.5*1.1 section, only the length changes.
Only major exception I can think of is Festool, who use a custom spiral cutting block, otherwise all the planer knives I've seen have been the same section (and mostly 82mm length).
You can take an oversized blade and shorten it, clamp it in a vice at the desired length and break the excess off (wear eye protection and gloves), but only in an emergency, best to get the right length in the first place.
Also, don't forget that you can turn the blades over and use the other side.
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