I think bigger is better where pens are concerned, but like gogburn said, I have some 16 x 16 pens and it doesn't seem to faze the deer.

Foolsparadise seems to have a pretty good handle on the subject too.

I think it can depend on how skittish the deer are at your place and how accustomed to fences. Our ranch is in an area that is largely unfenced, and when I put up small pens, the deer quit hitting those feeders except for an occasional visitor. Doubled the pen size to 8 panels and the deer were back. Not over-thinking it, just what worked at my place.

On the other hand, I live in a neighborhood that has tons of deer and axis that are very accustomed to people, and I think they would jump in a 4x4 pen if you threw some corn in it.