what is the gas port size for the MK18, i am getting a 300 blk barrel and having it cut to 10.5.

DD probably won't tell you. A lot of companies consider stuff like gas port size to be proprietary, so unless you get someone to pull their gasblock off and pin gauge their gas port, you probably won't get the port size.

Secondly, the port size for a 10.5" 5.56 and a 10.5" 300BLK will be an apples oranges comparisons due to the pressure differences. Also, I believe the 10.5" 300 BL will require a pistol length gas system.

So basically, DD prob wont give you the size, and if you did get it it wouldn't be of any help to you.

Guess I just have trail and error, I wanted to get rel my parts ready while I wait on my stamp.

Go to the 300 blk forum and ask there. Someone has to have all that info available for you