I just purchased new front ate rotors and 15mm ate pads. I want to change the calipers also. I know I need the ate style calipers but my question is the 49mm or 45mm caliper size. Which one is correct for the 15mm pads?

I'm sure others will know better than myself.
I'm guessing the 49 and 45mm measurements are referring to the OD of the caliper piston.
I would also guess that the 240D would be fitted with 45mm pistons since it is a base model.
Only way to know for sure would be to remove a caliper and measure the piston. Not the easiest if the car is needed for daily use, but also shouldn't take very long to remove a caliper, get your measurement, and re install it so the car can be used until you're ready to do the brake work.
Again, I'm sure others will know better. This is my best guess based on my experience.

Front caliper pistons are either 57 or 60 mm in diameter. The size is cast into the outer half of the caliper.

Of course.
I had forgotten that Ate did that.
Far easier than my method.