I ran into this issue with my group last game, and we've yet to come to a decision on how to handle this.
As background, my Samurai using a Naginata was enlarged, giving him the 10ft area he cannot attack, and then 20ft reach weapon area.
Another large enemy was straddling the inner and reach weapon zones as such-
.,,,,..
,,MM,,,
..mm..,
......,
..SS..,
..SS..,
S is my Samurai, mm is the half of the large monster in my 10' no hit zone, and MM is the other half of the monster, with , representing areas I could hit, and .. representing areas I could not.
Question 1. Can I even try attacking it?
Question 2. Does it provide cover to itself?
To keep the game moving, we ruled it provided cover to itself and I took a -4 to hit, but I was wondering what other opinions are on this matter.