To run at that size window, you had to click past the warning that said "Running at a width lower than 1024 pixels is not recommended for the best experience. Continue?"
Anything less than 1024 wide is basically unsupported at this point. My statistics are showing something like less than 0.5% of everybody running at 800xSomething these days.
Multiplier visible on top of commands
When describing problems, always mention your OS and game version (shown at the bottom of the title screen).
If your keyboard has USB or MIDI ports, there is a tremendously high chance (>99%) it will work with Synthesia. See what you'll need on the keyboards page.
If your keyboard has USB or MIDI ports, there is a tremendously high chance (>99%) it will work with Synthesia. See what you'll need on the keyboards page.