Okay, here's the deal. From a physics standpoint, changing the marble's size is easy. It's just a larger sphere. Realistically flowing water is almost impossible, even with current engines, let alone MB's engine.
The reason why is simply because of how many variables there are to work with. For a marble, you only need to keep track of six position variables: its position in each dimension and its rotation in each dimension. (MB requires seven because it uses angle-axis format, but that's another matter entirely.) Spheres are pretty simple. Now look at a glass of water. Notice how you can deform it into whatever shape you want. You would need hundreds of variables to get something that even looks decent. It's simply not practical with modern computing power.
The reason why is simply because of how many variables there are to work with. For a marble, you only need to keep track of six position variables: its position in each dimension and its rotation in each dimension. (MB requires seven because it uses angle-axis format, but that's another matter entirely.) Spheres are pretty simple. Now look at a glass of water. Notice how you can deform it into whatever shape you want. You would need hundreds of variables to get something that even looks decent. It's simply not practical with modern computing power.