20" models are almost non-existant in the US. Standard US monitor sizes are 13", 19", 25", and 27".
There were also 15" and 23" ones, but they aren't color and haven't been made for decades.
As far as I know Sega is the only company that ever used 20" monitors. I have one in the basement, it doesn't look any bigger than the 19" ones stacked all around it.