How bigger do you expect TLOU2 to be? it's important for the game to not lose its focus and pacing, I don't think it will be much bigger than U4.
But I'm also very excited for it, TLOU is all about the combat and AI, TLOU2 will probably have amazing sand box combat with various species and humans interacting in it and on a much bigger scale, better hardware can benefit such game tremendously, even more than what it can do to Uncharted.
ND already streched the PS3 with TLOU to its limits, it will be interesting to see what they will do on PS4 and more so, on PS4K :O
Not open world. More open levels though like Uncharted 4 is doing such as the Madagascar level. Still the same concept of being linear and having to get from point A to B but now you have a much better variation in different routes to get to point B. That is just my opinion though. I do trust Naughty Dog though to do whatever way they want cause I know I will still enjoy it.