The canvas size depends on what you want it to print or display at. You should make it at least that size, larger is always better but make sure you can shrink it easily.
You DO need to use at least 300ppi resolution to draw and print (if you want to publish though a company, you need to check their requirements, many of them need 60ppi.), you can cut it to 72 to post online.
Paper sizes at 300 or so DPI since I like that sort of stuff (Sai and most other gfx programs has a bunch of pre-set paper size canvases). If you ever want to print a landscape webcomic you could also do them in half pages ala Asterix. Two sideways A4's stacked ontop of one another is an A3. I think the same goes for US paper sizes but don't trust me on that since I usually do portrait format comics.