Just having a look at the box office charts and notice that all the Star Wars films are very Domestic (i.e. US) skewed. Just look at the % of total gross that is domestic for Star Wars (A New Hope) and the prequel trilogy compared to, say, Avatar. Star Wars always made around 45% or more of its gross in the USA, where as Avatar made only 27%.
So let's say Star Wars 7 were to smash all US box office records and make $1bn domestically, beating Avatar at $760m by over $240m... If it had 40% of its gross in the US (lower than all the previous Star Wars films) it would still "only" reach $2.5bn globally, failing to surpass Avatar. It would require just under 36% domestic to beat Avatar globally, which is easily possible but as I said it depends on Star Wars being as appealing globally as in the US. And that's assuming Star Wars is making $1bn in USA and absolutely demolishing Avatar and Titanic there... If it beats Avatar by, say, $40m in America and makes $800m there, it would need about 28% domestic gross, which is much lower than any Star Wars film so far.
That's why I voted it will be top in the USA (even Jurassic World and The Avengers weren't that far behind Avatar domestically, so Star Wars could probably top it) but not surpass it globally. But like I said, the global market has changed a lot and people in China could be even more hyped than the USA to get a new Star Wars for essentially the first time. Living in the UK and consuming media that is very dominated by the US, it is hard to get a feel for what the attitudes around the world will be.