Good question. I don’t really think there is a rule of thumb when it comes to the size of a canvas. The only question you got to ask yourself is whether you want your art to have the ability to be printed or whether you only want to share it with the internet. Assuming you only want to share it on the internet, any size will do. I work on a large canvas size and shrink the image down before uploading it anywhere because that is the method I am most comfortable with. I think the key here would just be to experiment with a bunch of different canvas sizes and figure out which one you like working with and which one gives you the results you like. I like large canvases because I like to have a lot of breathing room and I like having the ability to make large loose strokes and still be able to go into crisp detail work. Working on details is something that relaxes me so it is sort of natural that I would adopt a canvas size range that makes it so I can do that. That said I also embrace the downfalls to drawing on a bigger canvas which means sometimes I have to treat the picture I upload/export as a separate piece and fine-tune it to either 1. replicate the quality and feel of the original upon resize or 2. shine in it’s own way as a different variation of the piece I created on my digital canvas. But figuring out what canvas was best for me just took some experimentation drawing on a bunch of different resolutions until one specifically clicked with me. And even though I work on a large canvas, there is nothing wrong with working on a smaller canvas it just means some of the steps will be different. In the end, it all just really depends on what you are comfortable working at and what size makes it so you can create the art you want to create. Anyway, below are some of my pieces at their working full size resolution which may not be super helpful but I’ll jump at any chance to share my full res files. I hope this post helps you out a little.
(Oh and also if you are really drawing with true vectors (I’d assume you are using Illustrator or Mischief?) the size of your canvas shouldn’t matter as true vectors can be resized and exported to any size without losing any quality at all.)