283mm

[Ed Doe]

You can get whatever make of discs you want, they don't make any difference at all anyway, provided they are vented as per the oem discs.

406 discs will not fit as the hubs/calipers on the 406 are different offset to a 306.

Spend your money on decent pads, and make sure your fluid is renewed and the brakes are properly bled imo.
As has been said ferodo ds2500s and mintex 1144s are great; ferodo are slightly better for light track-use but more pricey. I've got 1144s and love them, and I have driven mine pretty hard in the past...