The recommended size of the canvas is not related to the program you draw it in, but where you want to publish it. 2000 pixels are 2000 pixels, whether in PS or sai or the web browser.
Basically, you should work at a larger size than you intend to publish, and at least as high dpi (dots per inch). The larger size ensures that tiny mistakes disappear when downsizing, and that you always have a sufficiently large and crisp image. You can always shrink a finished picture without quality loss, but enlarging it will only make it grainy or pixelated. The dpi is also something you can't increase after you've painted something, but you can lower to fit web formats.
AFAIK neither Tapas nor WT have height restrictions, so as long as you make your pages wider than the width restriction and set the dpi to at least 72 you're safe (web standard, but most work at 300-600 dpi from what I've seen, as do I). I'd at least double or triple the width, but maybe some veterans here have more concise recommendations.
Another thing to keep in mind is the ratio. If you intend to publish your comic in book form in the future, look up what format the printers demand and use that as a basis. Say they're printed in 21x30 cm size, that makes for a 7:10 width:height ratio. That means your canvas needs the same ratio, or the image will be distorted to fit the page or leave white space. What specific pixel size and dpi is appropriate for print is something the veterans of the forum know better than me