On my August 2002 RTW, I was in both the northern and southern hemispheres and experienced both winter and summer. I decided to take an 18 in rollaboard and a 25 in rollaboard (same design as 18 incher, but bigger). I carried two days of clothes, toiletries and my in-flight entertainment (books, laptop) in the 18 incher and checked the rest in the 25 incher. This way, should the checked bag get lost, I still had two days of clothes to get by with. On short trips, I make due with just the rollaboard, but once I start bringing extra shoes and replacement toiletries for a long trip, I need something bigger.
As for dress in F cabins, JohnAx's description would be fine, but I would think that a golf shirt would also be appropriate. I never wear the sleeper suits given-out as they are too hot for me, but, on a long flight, will change into a t-shirt and warm-up pants once airborne and change back prior to landing. I figure if everyone else is in PJs, my t-shirt and warm-up pants become much less uncouth.