I presume the OP is asking for a generic ideal 28b frame size, and not the size of the Manufrance Ideal model.

If so, I do not know what the preferred 28b frame size is, however for me, for grouse & woodcock, it would be as small as possible, yet strong enough to handle 2 1/2" RST lite loads. If you were shooting clays or ring necks, it would likely be different.

I have a Jeffery BLNE that weighs 4 lbs 5 oz., that frame measures as follows:
Breech width: 1.843"
Bar width @ the angle: 1.340"
Bar length (flat portion): 1.518"
Barrel flat length: 1.910"
Bar depth @ angle: .905"
Firing pin centers: .913"
The 26" barrels weigh 2 lbs 1.5 oz.
2 1/2" chambers, proofed for 5/8 oz.

I also have an A & F Zoli-Rizzini BLNE that weights 4 lbs 14.5 oz., that frame is even smaller, although it is a bit longer.
Breech width: 1.804"
Bar width @ angle: 1.360"
Bar length (flat portion): 1.868"
Barrel flat length: 2.350"
Bar depth @ angle: .830"
Firing pin centers: .860"
Those 26" barrels weigh 2 lbs even.
70 mm, Proof 0.910 KG.

These are both boxlocks, perhaps a trigger plate gun could be made with a smaller frame, although I have not seen one yet.

Has anyone measured a Dickinson 28b or .410?

BTW, I can reach around the breeches of both of these guns with my thumb & forefinger, with about 3/4" to spare, for whatever that is worth...

P.S.

In comparison, my lightest 20b is a Birmingham BLE, 2 1/2" 28" @ 5 lbs 2.2 oz. It measures:
Breech width: 1.994"
Bar width @ angle: 1.423"
Bar length (flat portion) 1.650"
Barrel flat length: 2.100"
Bar depth @ angle: .942"
Firing pin centers: .993"
Barrel weight: 2 lbs 5 oz., proofed for 7/8 oz in 1933.

Although this 20b is wonderfully trim & light, and it is not much larger in most dimensions than my 28 bores, there is a dramatic difference in hand, the 28s feel noticeably smaller and lighter.

Last edited by redoak; 10/13/19 10:16 PM. Reason: added 20b specs.