There are a couple of ways to resize the window:
If you are not currently in full screen mode, the ArtRage window has a standard titlebar upon which you should see the + and - buttons for sizing. The + button will size the window up to the maximum size the OS UI guidelines suggest it should be allowed to be (which is basically the height of the screen minus the height of the titlebar and the height of the dock if the doc is not set to auto hide).
If you are in Full Screen mode, the full screen toggle button can be found next to the close button on the ArtRage Menu Bar at the top of the canvas.
However, in both those cases the window size goes to 1024 x 768 or, if the screen is not large enough for that, full screen mode.
There's no resize handle in the OSX version because of the way the OS UI works. I tried it for a while but when you tell the window it can be resized, it forces a square 'tab' to appear at the bottom right of the window, which overlaps the colour picker's colour sample and gets in the way of ArtRage UI. The only other alternative seemed to be to make the window larger by adding scroll bars to the side and bottom so that the tab didn't overlap, but that was just ugly and as the ArtRage UI doesn't scroll anyway, another waste of space.
One plan I have noted down for a future revision is to add some kind of manual scaling system that handles it in ArtRage UI rather than relying on the OS. That way, the control wouldn't get in the way of the product and we have some choice as to how it's presented. Until then, I'm afraid the window size is fixed.