Hi, some basics:
* The most common keys (A-Z, 0-9, F keys, etc.) are 0.75 x 0.75 inches. These are called "1U" keys. One standard key unit wide.
* The modifier keys vary. Most to left and right of the space bar are 1.25U, or 1.5U, although 1U can sometime be found.
1.25U = 1U width x 1.25. So 0.75" * 1.25, whatever that comes out to be.
* If you look at top top row you'll count 13 1U keys, and an odd key at the far side. That's a 2U key for backspace
* That means the main block of keys is 15U wide.
* When I measure the distance between the middle of the "+" on top of the bottom-left keys I get about 1.25 times the distance from the center of the Z to X
* So, those 3 keys are likely 1.25U, that's common.
*About the same on the right.
* So, that's about 6 x 1.25U = 7.5U
* The space bar takes the rest, so 15U (from the 2nd step) less the 7.5U of modifiers should be around 7.5U of space bar.
7.5U
does not seem common to me. Someone should check my work.
Here's a page all about spacebars. Maybe that will help.
https://deskthority.net/wiki/Space_bar_dimensions