To answer the original question, no I do not have a preference.
The smallest brush I own is a Simpsons Major which has an 18mm knot and 41mm loft, the largest is a CH2 which is 27-28mm knot with 59mm loft. I don’t know how many I actually have but it is quite a few covering the Best, Super, Silvertip, 2 Band and boar types.
Each brush has a place and a reason why I like it. For instance when you hold the 18mm knot Simpsons Major in your hand for the first time you think “yeah right, this aint going to muchâ€Â, but the short 41mm loft gives it the stiffness it needs to produce almost comical amounts of lather from hard and soft soaps. I believe the “Wee Scott†is another very small brush with a similar characteristic; both are excellent travel brushes that are good enough to use in regular rotation.
My thoughts are in line with what has been said before; there is no specific knot and loft size that is better, it is the ratio between them that is important. The material used also affects the equation and is a variable that changes over time therefore the characteristic of some brushes changes as they bloom or as hairs split on boars.
I tend to try to match the brush to the soap, I have a few excellent brushes I reserve for creams and some I like to use on soaps. I tend to lean towards bigger brushes but I do rotate them round as I do my soaps, usually I have three or four brushes in rotation for a month or two then swap them for a different set.
Variety, as they say, is the spice of life.