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The ones that come up from the bottom and hold the exhaust in place on the flange are M10 X 1. This is out of a 99 parts manual though so as long as nothin changed there.
I popped the hood last night, stood on a milk crate and a 13mm socket fit perfect on the 2 bolts that attach the down pipe to the turbo flange. Either way, it's between 13mm-15mm & 7/16" - 5/8".