It took me a while to realize there was an issue in Chrome, as I instinctively use Ctrl + Mousewheel. Definitely looking forward to having this properly fixed, however in the interim thought I'd post a workaround here in case it's useful to anyone. I've placed *'s against the browsers I've tested and confirmed this to work in on Linux Mint/Ubuntu.
From the Keyboard
(Should work in Firefox*, Internet Explorer, Safari, Google Chrome*, Chromium*)
To zoom in, increasing the size, press and hold Ctrl while pressing “+” (plus).
To zoom out, decreasing the size, press and hold Ctrl while pressing “-” (minus).
To reset size back to normal, press and hold Ctrl while pressing “0″ (zero).
From the Mouse
(Should work in Firefox*, Internet Explorer, Safari, Google Chrome*, Chromium*)
If your mouse has a scroll wheel, press and hold Ctrl while scrolling up to zoom in and down to zoom out.