You might have to measure the screws by fitting one into the proper hole of a plastic sizing card.
You didn't say which Marlin model the base fits, or if you meant the size of the factory D/T'd receiver scope mount holes in whatever model Marlin - but, although most guns use a 6-48 screw(s), the centerfire Marlin lever actions, I've owned & worked on, all used the larger diameter 8-32 screws.
I can't say if other Marlin models use the same size screws - since I've never touched any, with regards to scoping.