Cs = y/(16*f*f) Ct = 3 * Cs (Approx)
Where Cs is the Saggital coma and Ct is the tangential coma.
y is the linear distance off axis f is the focal ratio
As you can see, the mirror diameter does not enter into the equation.
To quote from _ manual of advanced celestial photography_ Page 18
" Thus a 6 Inch f/6 and a 100 Inch f/6 both have the same 1 Inch diameter coma-free photographic field."
-Alan Kilian