So I started a hobby of writing and drawing manga, scanning them, polishing on the computer, etc...

I just wanna ask what paper size do the doujinshi artists use for their doujinshi.

I want to print one copy.

5½ by 7¾ usually for the manga volumes you would find at like a Barnes&Nobles or something

Alright, thanks.

If you're wanting to draw and scan it as its original size, then yeah, they are usually 5 1/2 by 7 3/4. However, the art teacher that I had in high school was a professional freelance comic artist (works for Marvel, DC, etc. doing coloring and whatnot on a regular basis) and when he was drawing pages for his personal comic, he drew on pages that were almost twice the size of 8 x 10 printing paper (either A3 which is 11.7 x 16.5 or A2 which is 16.5 x 23.4, probably A3 so it's not too large). I think he did this so he can really focus on the fine details of the line work without having to get ridiculously fine pens. I'm sure you'd be fine using normal paper and resizing it to standard manga size or even just using manga sized paper itself, but what I mentioned before was what I observed from a professional. You should do whatever works best for you.