Does a plant grow bigger if watered with milk or water?

The only way to be sure is to try it. Milk is somewhat acidic and rots though. It depends on the kind of plant, drainage etc. many conditions.

Water! Plants grow not directly because of what you put in them, but from the sugars that they make during PHOTOSYNTHESIS. The plants take the carbon dioxide from the air, as well as sunlight and WATER, and produce sugars which they use to grow. Not MILK. WATER. Plants have little use for calcium.

Milk will kill most plants.

water!

Neither, in each case the plant will grow as big as its genetic complement dictates. Plants will only really take up the water from the milk, it is possible that they will absorb some of the calcium but this would not effect the growth - the plant wont use it for energy and can only use what it needs to and enough should be in your medium. The protein in the milk will, however, sit around in the growth media and feed bacteria, and fungi if your experiment isn't sterile, so in practice the milk feed plant may actually suffer, if some of these microbes hurt it that is.