Does anyone know how the Doctor determines how much of the stomach is removed during sleeve surgery?...Is it a standard amount? Does it depend on the size? I've been continuously thinking about this while I wait for my big day...

I think an anesthesia almost accidentally used a 32 (for the bypass) on a sleeve (which is a 36). And if that would have happened it could cause leaks and be rather dangerous for the patient. So he standardized it to use the same size bougie for all patients and reduce the chances for a mistake.
Theres small stuff like that which happens ALL the time that transforms the way that your care is performed.

Some docs (mine) use 32 for a sleeve.

New research shows smaller bougie does have higher complication risk (slight) and no higher long term weight loss. So that may have been when the change happened, when that research came out. But I'm not sure a 32 is truly "dangerous" just has a higher complication risk.