Just looked on wiki the stadium holds 55k for concerts. Looking at that seating plan it’s got to be half that for the u2 gig
The seating plan looks like they're selling just over half the stadium (which is symmetrical), so I'd say over 28,000 capacity. Add the GA on that pretty large field & we're well over 30,000.
Can't fill the field totally, as GA on the sides will have a horrible view. The field is very wide, end zone to end zone. Fitting a rugby field, which is 100m, plus the in-goal areas, plus more to spare. I'd say a total of 130m. The screen is under half that, 61m. So that seating plan doesn't look accurate, the stage will look much smaller in real life. This is a more accurate mock-up.
Note: looking at the Optus Stadium (Perth) seating plan, & at what tickets have not been offered for sale in Sydney, it now looks like they've split the sound/video booth into two, pushing them to the corners. Hopefully, as there's now two, they're only single storey this time.
So GA at the sides would have a horrible view (so expect reduced capacity). As will some of the blue Cat 2 seats on the sides, they will suck! Conversely, the front-on Cat 1 seats will have an amazing view. They'll be very close, not much more than 50m away, half the distance of the front-on end-of-field in the usual layout. The CAT 3-6 behind then are where the bargains will be found, especially if there's dynamic price reductions at a later date. Although there's a chance the sound could be pretty bad up there.
My mistake 
Zooropa for the first time! And what they did play off of NOLOTH. 
You can’t take it with you when it’s over!
