Good question, Ed. While Firefox 64bit works very well, it has those annoying little info popups of shift or Control or any modifier keys while using the tablet. No idea how to turn that off, yet?!
There are a number of open source browsers and the likes that are working well. Last, but not least, there's Node.js, which has the chromium engine. I may actually prepare a stand-alone release with this.
Maximum size... if I rework undo a bit, I imagine it could go up to 10000, depending on your card. I'm literally only rendering where you're drawing, unless you change lighting or do any compositing, of course. So it's quite fast even into highest resolutions. However, with fluids this may change, but I'm still thinking about ways to make adaptive fluid rendering possible, which would again may allow for great speed at highest resolutions.
On my poor little GTX 970 I currently can still comfortable paint on 4000x5000, while I can hear the fans going. BUT I haven't optimized in a while and got a lot going on. It will be good, though!