You also needn't worry about how big it grows - NetApp controllers can handle a lot per volume. In a 4 node NAS cluster I have 1 SVM that contains 4.8 billion files - the largest volume is at 175 million or so.
The implication of a large file system in general is that if your users are going to put 100K files in a single directory, they'll notice a performance hit when they try to access that directory, but that is more due to general file system access issues rather than a property of Data OnTap itself. So long as the volume in question has a good directory structure to separate out all those files, you shouldn't notice anything (other than the need to increase inode count from time to time).
Bob