a 3 inch recessed light should fit in there, I used one like this in a cherry soffit built above our kitchen sink:
http://www.lowes.com/ProductDisplay?partNumber=520454-43921-MQTL3001-LED6KAOW&langId=-1&storeId=10151&productId=50121571&catalogId=10051&cmRelshp=rel&rel=nofollow&cId=PDIO1
I like...