What is the size of the filler screws used to fill the scope mounting holds on the Marlin 336?

6 by 48 or 8 by 40. Probably the 6 by 48 as there the most popular scope base screw size.

Thanks for the info. I will just order both. I know I will use them sometime in the future.

For the scope mounts, most Marlins use 8-40.