Biggest Dirt Bike
It depends how you define 'dirt bike'. If you mean strictly motocross race bikes (the CR / KX / YZ / RM lines, etc, and their 4 stroke counterparts), then yes, in the last 10 years, the 450s are the biggest they've made. The 450s are all the four stroke 'progressions' of the 250cc two stroke race bikes (and the 250 4 strokes are like the 125 2 strokes).
2004 (11 years ago.:) was the last year there was a 500cc motocross bike. Kawasaki made the KX500 then, and this was its last year in production. Kawasaki and Honda had both made 500s (Honda's was the CR500) for a long, long time. The demand just wasn't there though, esp when the 4 strokes came around and really started to take over.
If you're talking dirt-bike as in MX AND off-road/ enduro, then there are a few that are in the 500cc ranges in the latter grouping. KTM makes a 525, 505, and of course Honda makes the XR650R.
If you're thinking 'biggest' means 'most powerful', however, that's not always the case. Unless you're talking about the two stroke 500's, as those things were BEASTS:)