I guess you must be referring to the hole in the upper oil pan.
The hole itself is about 24mm across, you will never get a 24mm drill bit but I drilled mine out to about 21mm using a unibit, just make sure the edges are nice and smooth so they dont obstruct the oil flow.
On either side of the drain hole are the marks for the bolts to hold the oil return fitting on. You dont need to drill these all the way through, just open them up a bit to 6.75mm and then tap them for M8x1.25 bolts, it will be nice and secure then.
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