Is It Washington 1 Cent Scott #544

I have the exact same story but mine is numismatic not philatelic

And no I dont have a scott catalog. Because I just started learning about them. I have collected more like numismatic and WW2 stuff and now I want to start learning more about stamps.

Kevin-Eric, where did you read to measure your stamp with a ruler to identify it's identity? Everyone who thinks their common stamp is a rarity says something about the size of the design. If you don't have a catalog, where did you get the idea to do that? I am curious as to where this misconception is coming from.

Dubinweb.com

Now I learned that what is a perf 10 and 11. Its those teeth what I have to look for too.

So it is scott #543 not scott #544

It says that my stamps design has to be 19mm wide and 22,5 mm long. I dont know how to measure it because I was learning more about numismatic and I thought that its the same way how to do it.

And how to distinguish flat plate printing, rotary press printing and offset printing.

Measurement is used only by experts to confirm an identification after all other factors have been considered, and measurement by comparison to a known flat plate stamp is essential to identify the rotary press rarities. As others have said, forget about rulers. Unfortunately the Scott catalog gives some people the impression that measurement is all you need, and various websites (such as the one you quote) have propagated this, to the detriment of the hobby. It has misled many a collector and it would be best to ignore all measurement information. Just pretend it isn't printed there, and learn all the other identifying characteristics.