It depends but, generally speaking, it will be. The iOS operating system takes up around 1.5GB, and a single AAC app could take up a similar amount again (AAC apps tend to be large because things like synthetic voices and large symbol sets take up a lot of storage space). So if you’re using a 16GB Pad with a single AAC app you’ll probably have at least 12-13GB spare for everything else. In most situations this will be fine.
You may want to get a larger device if:
You are using the iPad for assessments or therapy purposes and intend to download a lot of different AAC apps and therapy games.
You are using the iPad yourself but want to use it to take photos, videos etc as well as to communicate.
You are likely to download a lot of additional content to the iPad (music, films, other apps, etc).