I use A3 (3508x4961 pixel) and 300ppi resolution as a standard. It's enough big to do almost anything and enough small to use. If you can it's better do too big than too small, because you can reduce it's size afterwards and it looks good.
It depends on what you want to pain. I usually make my canvas size somewhere over 1000... sometimes larger. So I would say something like 2000 x 1500 or something like that... you can always adjust it you know. It's better to make it larger if you want to get really detailed, but it also makes your computer load a bit slower (or at least it does for mine) And then yes, I also use 300 ppi... It took me a while to figure out that that was the resolution, so if your program doesn't say ppi, or dpi or whatever... just set the resolution to 300.
Of course, I should probably mention that I use Gimp a lot. I like using Photo shop... but for some reason I can't get my software to work on my computer and Gimp is so much cheaper than re-buying photo shop. ^_^
Depends on where you want to use it what size you want etc... For example I start my work most of the time as an A4 in 600 dpi (which works out to 4961 x 7016) then I reduce to 72dpi when I upload to deviantart, if however i wanted to print that artwork I would print from the 600 dpi file I should also mention that 600 dpi is actually more than neccesary when printing most conventional printing is done at 300 dpi. I use 600 because my computer can handle it and it allows me to enlarge my work when printing ... to a point