Hi Nathalia,
It's not that you need to stick to lower resolution because you are a beginner (more, that painting on higher resolution textures is slower and can get really sluggish (in Blender at the moment) and it takes up more memory), but all the rest of your assumptions are pretty much spot on, I'd say..
It will depend on what you need. If you're going for a highly detailed model, you'd probably end up using 4K or even 8K textures, it will take a toll on Blender's performance. If you're doing low poly then lower resolutions. I guess the most standard is 2K, that is usually what you'll need most of the time.
The scale is not important, what is is important is how far or close is the object to the camera and what's the resolution of your render. If it's far from the camera, it will take up less pixels on the screen so you'll be fine with lower resolutions, but if it's very close to the camera, then it needs a lot of pixels to look good so you'll need a lot of resolution.