If I start to play around with learning Adobe because I’m interested in stuff with this game and making it somewhat to scale or whatever… is there a particular setting in Adobe you guys are using when setting up the object sprites for the sprite sheet: is it pixels, cm, etc?
Sprite sheets are in PX. Base two multiplies, so 128px, 256px, 512px. Largest in Unity is 8192px but they I believe are not always supported on lower graphics cards. 4096px would be as large as I’d go, but larger can be done with other methods.
Having said that though, how large is a pixel? Every screen is different PX density hence the PPI value. Don’t fall for the myth of 72dpi screen resolution as that is just pure myth.
Hope that helps?
EDIT: Having re-read your original question, I don’t know.
Adobe works with vectors, so the pixels are mostly irrelevant I guess… ok, nevermind. But the sprite sheet pixel count helps huge. How many pixels then is a square on one of the sprite sheets
It’s not quite that simple. PX counts in Illustrator are not the same in ID or PS. It’s a horrible mess. You’ll find lots of articles about it if you do a search about switching artwork between the three. I’ve managed a good work-around now though :).
Sprite sheets to PX square are all very different. You only need to look inside a JSON file for the aircraft to see that in action. 
I’m a beginner in Unity and so cannot help much in that regard I’m afraid.