Can anyone tell me what the size of the m40 nut is?

There is no m40 nut size in the national standard. You can only refer to the size of the M39 nut or the size of the M42 nut. The thread is made of M40. Of course, the specific size is determined by negotiation between the supplier and the buyer. It is best to check the relevant information yourself. What you get online is always fake. If you encounter someone who pretends to understand and respond casually, it is completely fake or deceptive and will directly mislead you and even harm you.