Thermostat housing bolts size needed

Part number is 06104364AA. You are correct. Finding the size is just about impossible.

Easiest is to just find them in a salvage yard. Besides getting the correct size, if the holes are drilled into the coolant passage, the bolts will be specially treated for corrosion resistance. Standard bolts can cause "galvanic action", (corrosion), when two dissimilar metals come in contact with an acid. Acids normally develop in engine coolant over time. Antifreeze has numerous additives, one of which is a corrosion inhibitor, but those additives wear out in about two years. If the cooling system is neglected, those two or three bolts could become stuck and break off next time they need to be removed.

You will also usually find a stepped shoulder under the head of the bolt. Its purpose is to center the thermostat housing over the hole in the cylinder head or intake manifold. Regular bolts won't do that, possibly resulting in the rubber seal being pinched under the housing. The housing won't sit flat, the seal will leak, and the ears on the housing could break if the bolts are overt tightened.

Torque spec is 20 foot-pounds for these bolts.