Kailh chocs has 18x17mm units. This isn't too small.
Just to clarify: Kailh's
keycaps for Kailh Choc has those units. Mass-produced keyboards with those switches have keys that are standard 3/4" (19.05mm) square. Keyboards that fit both Kailh and Cherry MX also have standard units.
When designing for Kailh's keycaps, you must also know that they are nonstandard in another way: They were made for 0.5 unit uniform stagger, so some keys are in non-standard number of units wide. For instance, Caps Lock and Enter are 2u instead of 1.75u and 2.25u, and both Shift keys are 2.5u instead of 2.25u and 2.75u.
Of course, Kailh Choc will also need special stabilizers, but those are available.