This may seem like a ridiculous complaint, but I actually ran into a similar limit in Chrome with respect to the width of a canvas and had to pull all kinds of nasty tricks to get around it.
I tried to render a piece of music horizontally on one canvas. That didn't work past a very low limit. In the end I chopped it up into multiple canvases and scroll at most two of them at the time (there is some risk of artifacts at the point where they meet, unfortunately).
Mobile Safari lets canvas get bigger, it just gets ridiculously slow to work with