forums are slow but you usually get good answers here. reason i saw your post on github so fast was because I get email notifications for github posts.
Okay, so lemme answer the questions you have and try to answer the questions you should have asked
This is a in development feature. It is not currently merged into the nightly builds. You can use the feature in bleeding edge if you want to try it out. Simply install bleeding edge (or if you don't like installers, download the full.nupkg and rename the .nupkg to .zip and you'll be able to extract it)
In order to use the feature you need to edit the qt-config.ini settings. In the bleeding edge version of citra, the qt ini settings will have a bunch of settings like
custom_top_right and you can set them to the pixel values you want. This is not very user friendly, and its really made for people who can figure it out and have them share their config to users that aren't as citra savvy. In the future, I have some ideas to make it easy for people to edit their custom layouts, but they'll take a lot of code to do that.
Short answer: right now you don't. Custom hotkeys just aren't a thing in citra yet, but with a little code that can change. If anyone is looking for an easy task for a first time contribution that is something that shouldn't be too hard to do and should be relatively quick to merge imo.
You dont. Well, if you don't already know how to, then its not worth teaching you how to as its really something for the developers to do and not for users. Thats why I created bleeding edge, so users don't have to learn how to merge and build. If you really want to, you'll need to understand git and how to compile citra (there are instructions for it on the wiki)
because you are downloading the code for citra, not the executable for it.
So what should you do? Download bleeding edge, either the installer or the .nupkg (which is really just a zip file) and run that one. Then you can edit the settings.ini file in
C:\USERNAME\AppData\Roaming\Citra\config\ and change the
custom_* options to whatever dimensions you want.