With very few (if any) exceptions, Left-hand threads are identical dimensionally to the same Right-hand thread.

Whatever dimensions a right-handed thread has will be the same for the left-handed thread.


==edit==
sorry, I missed the intent of the question.

M6 x .5 will follow the same rule of thumb as other threads for internal threads -
6mm(OD) - 0.5mm (pitch) = 5.5mm tap drill.

Here is a good reference.