How large is 300px?

The physical size of a pixel is going to depend on the resolution of the device displaying it. Resolution is typically measured in Dots per Inch (DPI). The general formula to convert from pixels to inches is: Pixels / DPI = Size in Inches.

For example, at 75 DPI (typical CRT display) 300px would be 4 inches; while at 150DPI (typical cell phone display) 300px would be 2 inches.