If there will only be two of you, go with a 6". I made the mistake of ordering an 8" this past Dec for DW's bday, and it was a lot of cake for us to force down over a few days. The 6" was around $25, I think. The 8" was closer to $40. The bakery asked what kind of cake, recommended a filling, and I chose the icing (buttercream). I don't know if it was an odd day, but the buttercream icing was heavy on the butter and light on the sugar.
I also ordered a Mickey head cake a couple of years ago. It was a lot of cake too. But the decoration was too cool to not do at least once, even if we did end up throwing some of it away. I have a pic of it somewhere.