The monitor of my PC measures 19.5 inches and it's optimal resolution in Windows is 1366x768 I can use MUGEN in HD (1280x720) but not at Full HD specs (1920x1080) so how many inches does a monitor have to measure to have the optimal 1920x1080 resolution in Windows? Also how many inches does a monitor have to measure to have the optimal 2560 x 1440 resolution in Windows?

I think i know your issue, but if I explained it to you now it would make no sense at all.
The reason is because, you are asking the wrong questions.
So I think first, lets start you off here with some homework.









After watching these you will understand the basic 101 of screens, resolutions, aspect ratios and DPI.
Once you understand all of these, then try reask your question.

Good luck and happy research

yeah like the screen size doesnt have anything to do with it bro.
shit look at smartphones now that have 4k displays n shit.
but i think the bigger screen is the better for seeing detail if you are going over 1080p bet.

People here have misunderstood me. I have an operating system whose recommended resolution is 1366x768. If I select another resolution such as 1920x1080 all the pixels have to be compacted so they can fit on the monitor in turn. Come on, my phone's resolution is 720x1440.

Now that I have had the opportunity to use a new computer with a 24-inch monitor, the recommended resolution is precisely 1920x1080. Despite being a slightly larger monitor than the one I previously mentioned. It is evident that everything looks a little smaller due to what I already commented.

I understand but also i will explain to you the same as the others my friend.
the inches size of your monitor does not have anything to do with your optimal resolution.
You can scale your text and icons and all things on your windows operating system if they look too big or small when you use HD resolution.
You can google "windows desktop scale settings" and maybe you will find some success on this topic my friend