Vignetting and baffle diameter is merely a matter of selecting the baffle size that vignettes your widest field by zero percent.
The secondary itself will vignette some (unless it's chosen for 100% illumination over the entire chip), so why add to that with a tube baffle?
If you look at the baffle diameter, a simple tangent function will suffice:
The clearance between the mirror and the tube should be equal to 1/2 the field size in degrees of your widest field.
So, the adjacent side is equal to the edge of the mirror up to the tube opening.
The opposite side is the distance from the mirror edge to the tube (or baffle)
And the angle is the angle between a vertical line from the mirror edge and a straight line from the mirror edge to the inside lip of the front of the tube (or baffle).
You can easily see that the farther an aperture is from the mirror, the larger it needs to be in inside diameter to avoid vignetting.
I'll use my 12.5" as an example:
Maximum field = 1.3 degrees
so the angle between mirror edge and inside diameter at the top of the UTA will be 0.65 degrees.
Distance from mirror edge to lip of tube (rounded off for simplicity) = 60 inches.
So, tan .65= x/60
where X = mirror to tube I.D. (or baffle I.D.) the actual I.D. of the baffle or UTA = Mirror diameter + 2X.
On my scope, X = 0.68", so an I.D. of 12.5 + .68 + .68 = 13.86" would work with that field size.
The mirror-to-UTA lip is actually slightly less than 60" on my scope, and the New Method of collimation slightly tips the optical axis toward the focuser anyway, and i might use an eyepiece with a larger field stop (like a 31 Nagler) so a slightly larger I.D. of the UTA is desirable. In fact, it is 14.25".
So how large would a baffle be, placed at distance X from the mirror.
Well, at the mirror, a baffle can = the mirror size without vignetting.
At 60", it needs to be 13.9"
Which means that 30" off the mirror, it needs to be (12.5" + 13.9")/2 in size, or 13.2"
Similarly, you could easily figure out the baffle diameter at any distance from the mirror.
If your scope is a full tube type, you may discover that the tube I.D. itself vignettes the field of view. That is fairly typical.
In that case, you don't need baffles but external rings to surround the tube (and be bolted to it) to provide greater stiffness to the tube.
Alternately, an additional piece of tube could be attached to the tube either internally or externally to increase the wall thickness of the tube.