The size of the opening should fit tightly to the light fixture casing, but the real issue is what type of recessed light fixture you install. There should be an airtight barrier designed to prevent moisture from the house from flowing into the cold attic and causing condensation problems. Some recessed light fixtures will overheat if you insulate them, and most recessed light fixtures leak tremendous quantities of air into the attic-enough to potentially rot out your roof. For years I have recommended not using recessed light fixtures in an insulated attic; track lighting is a better option. But there are new airtight recessed light fixtures specifically designed to be buried in insulation and the casing has no holes in the inner wall. To make the job complete, we come back to your original question, the size of the opening. Cut it tight to the casing and then use the special air-sealing gasket that comes with the airtight fixtures to seal the inner casing to the drywall, closing out all air leakage into the attic. As for the junction box, most electrical codes require that junction boxes be accessible. Accessible from the attic is adequate, and the recessed lights come with a junction box built in.