Yo! welcome aboard!
The A sizes are usually good references, the higher the number, the lower the resolution, with A6 being 1748 x 1240 px
I always wondered about resolution when starting out as well. A6/A5 is good enough for studies, since you're usually just practicing and resolution quality is kind of whatever, but as you progress eventually you will want to jump to A4 and A3.
It should be a need-to-basis though, since higher resolution means more RAM and space consumption, and if you don't need to, it's better to keep it low.
I usually work at 300 dpi. it's usually a good spot for general stuff.
As @Pattu_w mentioned, it also depends a lot with what your hardware can handle and how much space you have available. A6/A5 will usually hit a 20-40 Mb size, and it just goes up from there, with some A3 taking up to 400Mb or 500 Mb (In clip studio). Of course it depends on the complexiy of the picture, a blank canvas won't be as heavy as a full detailed picture with many layers.
Also keep in mind you can change the resolution of your canvas after you started. It's not a one time setting.
I recently started to work with A6/A5 as a base, and after the initial sketch and flats are done, I'll bump up the resolution, and if I need even further detail, I'll bump it up again.
