What do you need dimensions for? That really is an odd question =)
Don't need one to answer it though, you just need some basic image software, something like ImageJ will work, anything that can pixel count an on screen drawn line.
Measure the 5 inch floppy drive bay, use the pixel count from edge to edge to calibrate the pixel to inch to determine how many pixels equal an inch and calculate from there.
All you need to know to do something like this with a random image is to have a known dimension, be able to measure the pixel distance, and be sure it's in the same parallax line to avoid distortion. Even that can be accounted for but it's more complicated.