Technically for computer screens you can use any resolution you want. 640x480 and 320x240 have become quite common sizes. I suspect it's because they both have the same width to height ratio as a computer monitor and also in the old days the monitor resolutions actually were 640x480. Monitor resolutions have increased over time (mine right now goes up to 1920x1440) but you've got to remember that the larger the dimensions of your animation, the greater the demands are on the computer processor speed, hard drive access speed, and VideoRAM. Therefore creating animations at 1920x1440 is probably not a good idea if you want people who have anything but the absolute latest, greatest supercomputers to be able to watch your animation in real time. I go up to 640x480 but no larger unless I'm creating for TV/DVD/HD movie.

Before they made me they broke the mold!
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