Is there a limit to the size of a family file I can put on the iPad?
Reunion does not impose a limitation on the family file size or number of people.
Most features (like navigating from one person to another, editing data, adding new names, etc.) are not affected by the size of the family file.
The main feature that will become slower with large numbers of people is the sidebar lists of People and Places. Performance in the sidebar begins to degrade in family files exceeding approximately 10,000 people. The processor in the iPad is not nearly as fast as the processor in your Mac.
If you have a very large family file, one work-around is to use the Find feature. I.e. you can search for a particular person or last name instead of using the People list containing the entire list of names.
Perhaps the best work-around is to send a marked subset of your family file to the iPad, rather than the entire family file. For example, you can use the Mark > Mark Groups feature in Reunion to mark a subset group of individuals in your family file and upload them to the iPad. Information about using the Mark Groups feature can be found by searching your Reunion manual for "mark groups" (without the quotes).
Of course, you can try uploading your entire family file and see if the speed of the People list is a problem for you. The file on the iPad can be deleted if you are not happy with the performance.