How da fuck do I get an accurate A4 sized canvas on PSCS? I think my drop down option might be a bit messed up. xD

I use an old iMac.

I have an image quite a bit smaller than what I would've thought to be A4, 570x776 px, I choose the A4 on the drop down and it's absolutely MASSIVE.

When I put the image on it at 16%, it's about 70x100 in the middle of the canvas, the rest is all white.

It's seriously huge. At 100% the image in normal size but the canvas is still like woah.

Any ideas? Is there a set amount of pixels for A4 instead of mm's?

I don't want the art to be all pixellated. D;

If the resolution is high (like 300), that would account for the huge size.

If it's just for internet usage, then put it at 72 and it should be sweet?

So what if I were to print it out if it was at 72? Would it still be A4 size (Fill up the whole page)?

[edit] If I wanted to save a PS document so I could then go on a windows and edit it in MS publisher, which file do I save it as?