As the name would suggest the Hamilton Khaki Field Mechanical 38mm
link is you guessed it 38mm in diameter.
link There's a 42mm version of the Khaki Field Mechanical and an automatic version
link with different dial styling and configuration but even on my 7.25" wrist and tall frame I'm dead set on the opinion this is the platonic ideal of what a field watch should be. The original Khaki watches were 33mm but modern tastes have changed and 38mm feels like the perfect sweet spot.