plate sizes are in inches or cm, depending on country of origin. unlike film dimensions 4 inches by 5 inches is 4 inches by 5 inches , unlike film which 4 inches by 5 inches is about 1/16" smaller all the way around ( 3 15/16 inches by 4 15/16 inches ).
That depends on the country but in the US full plate (4/4) is 6.5" x 8.5". Quarter-plate (1/4) is 3.25" x 4.25", sixth-plate is 2.75" x 3.25" and half-plate is 4.25" x 5.5". Eight-plate is 2.125" x 3.25" and ninth-plate is 2.0" x 2.5". Other standard glass plate sizes would be familiar modern formats such as 4" x 5", 5" x 7", 8" x 10", and 10"x12". I think 18"x22" was the historic "mammoth plate" size (although I'm not sure about that).
I noticed you also recently queried about old Darlot lenses. If you are thinking of teaming up a historic lens formula with a certain plate size, you should be aware that earlier Petzval lenses had less coverage than modern-day lenses. For example, for full-plate you might want a 12" or 13" Petzval lens in contrast to a modern 12" lens being the normal focal length for 8"x10" film cameras. (Coincidentally, I have a Dallmeyer 3A Patent Portrait lens for full-plate listed in the classifieds if you have any interest in that.)
Unfortunately, there are no Darlot catalogs there. However, someone is selling CDs of old Darlot and Dallmeyer catalogs on eBay and you can also find the "Lens Collectors' Vade-Mecum" CD on sale there often. I have some reference materials so if you can give me an idea of what you might be looking for, perhaps I can find some info for you too. Unfortunately, I have nothing on Darlot lenses.
Get a full plate camera, and we can swap lenses. I have two nice lenses that Jim Galli is keeping safe for me while he tests them out. So far word is they are great. I just need to restore my Full Plate camera from 1898. First would be the bellows, then the nice satin snow ground glass, then maybe some wood refinishing.