I believe the internal dimension of a conventional 1-1/8" head tube is 34mm. of course, the specific diameter is smaller than that so that a headset cup will fit tight, but it's nominally 34mm.

Park Tool has a good article about SHIS that gets more specific. they say that a head tube for 1-1/8" should be 33.8–33.95.



Cane Creek developed the SHIS, so go there for the original information.

the bottom of the fork steerer tube for that sort of headset would be 30mm. that's where the crown race presses on. the fork steerer should be 1-1/8" or 28.6mm, but the crown race interface is a little bigger. those are all approximate numbers. you'd want a more precise measurement for frame building.