The ideal F cimbasso bore size...

[imperialbari]

Part of the equation is about the valve system. Rotary valves need a larger bore to avoid stuffiness. Not that I would state bigger bore always being better. Loosing any bite to the sound would forfeit the purpose of a bass for the trombone section.

And then the player type also matters. As I read and hear bloke, he is a high pressure player, whereas I was a low pressure player, which calls for a larger bore.

Klaus

[Bob Kolada]

Fwiw, I had a small bore 3 valve Eb built up a while ago as part of a trade to serve as a valve alternate to a Bb slide bass trombone. It has a .562 valve set and a really good low C-Bb.

[Sam Gnagey]

The basic structure should follow the plan that 1/3 of the length of the open bugle is in conical section whilst 2/3 is cylindrical. I don't think it matters where the valve cluster is within that cylindrical section as long as the lead pipe is near the same proportional length as on the typical modern symphonic trombone. That's the way I build them. I prefer the piston .687 valve cluster for the F and Eb instruments.
Sam

[J.c. Sherman]

I see the cimbasso as an instrument that bridges the gap between a distinctly trombone sound and a distinctly tuba sound... so I figure MY ideal bore would be on the low side, between .600" and .650".

Nope... it's a trombone. It must sound like one. .562-.656 would be fine in different configurations (my slide instrument is .562) but with a tuba diameter mouthpiece, I'd lean more to a .689.

$.02

[joh_tuba]

I have no opinion or experience on the topic. BUT, thought putting together data points into one post might prove helpful?

Cerveny 576 14.2mm/.559" (presumably designed to suit trombonists)
Haag Cimbasso 16.5mm/.650" (arguably blowing more open than the bore suggests due to Haag valves)
Cerveny 652 18.2mm/.717" (presumably designed to suit tubists)
MW 41 18.5mm/.728
G&P 19mm/.748" (piston valves, according to horn guys website this is the most 'tuba like' cimbasso they carry)