Check out the other free web store games, and see if there is any restrictions on them.
For safe desktop resolutions, 640x480 not supported by some modern cards.
I would include:
4:3 aspect ratio for old computers and monitors:
640x480 some modern cards emulate this full screen, so it is not really optimal for gaming.
1024x768 For some HDTV's this will have a faster refresh rate for computer and video games. they range from 62hz all the way up to 85hz.
If you want to go widescreen add the additional:
1280x720 This is also kown as 720p for cheap HDTV's, iirc this is all nintendo Wii displays in.
1920x1080 This is 1080p. Most modern HDTV's will display this.
I am not really sure what is safe for Europe or asia. I think they have a 1366x1024 which is quite common.