UGGWomen's Classic Ultra Mini Boot

I need to know some information, I’m normally 71/2 and I interested to buy the classic ultra mini so what size is recomendable to buy it?

Should I size down I'm a 61/2 but all other Uggs i can get by with a 7?

Are the Chesnut boots really in the color "chesnut"? I own a pair of chesnut uggs and the pictures posted look much lighter than what they should be.

Who is the seller? Are these authentic Uggs?