No, but it's pretty big. The Canvas repeats at the edges and at the extremes of zoom (the entire canvas is contained within every pixel), so determining a true size is difficult. At a resolution 96 dpi, a single level of zoom would cover about 500,000 square miles, about twice the size of France. And there about 4 billion levels of zoom. Said another way, there's approximately enough space in the Canvas that if it were scaled to the size of the human body, each pixel would be as big as an atom.

If course, we don't have enough storage available for all of that data, but we'll cross that bridge when we come to it.

There are currently no plans to support Internet Explorer. For now, it's convenient to limit how many people can use the system. Eventually, I expect Internet Explorer to support HTML 5, which includes the graphics language that the Canvas relies on.