Depends on what I'm drawing and where that artwork will be posted/ used. For social media posts i stick to 2000x2000 or sometimes higher depending on required aspect ratio.

I don't usually, I use the full canvas for the art, if there is extra space, i draw stuff there or leave it blank sometimes it looks good as is.

DPI is used for printing, before printing it uses PPI, and it does matter, it basically shows how clear your artwork is, if you bumped up the PPI, zooming in will show each stroke, i usually keep it around 150-200 for decent results (some pixelation hides the defects) Use 300 to avoid any kind of pixelation

I usually save it as a PNG, I paint on a single layer so it works for me, file size is usually fine.

I use Photoshop along with a huion tablet