In another life I used the late seventies through to late eighties DeVere enlargers in a commercial environment, pretty much all of them kitted out with what your model is, the cream brown version.
Within reason we never used anything else except the 4x5" mixing box, the exception was usually when enlarging 35mm over about 1m on the long side, which is a pretty big print if printed whole.
In my own current darkroom I have exactly what you have and I keep the 4x5" mixer box in always for 4x5", 35mm, 6x7 and 6x9 negatives.
From memory, the difference is about ¼ of a stop in light gain by switching to the middle sized light box and about 1/3 of a stop in light gain for the 35mm mixer box over the 4x5" mixer box. The 35mm mixer box was (I think) a special order in Australia.
Mick.