Objectively, all the woods and Garden stage headliners are hammersmith apollo size, with Idles being arena size ( and headlining the second stage at glastonbury)
Aside from that, lower down the line up, top 3/4 are barbican/roundhouse size roughly.
People may not like the line up on an artistic basis ( i think its great), but its clearly one of their biggest line ups commercially. Id be very surprised if it didn't sell out well before the weekend.