I can't give you an official answer, but I have 588 in my family tree right now, and there is no noticable slowdown in the responsiveness of the program. My impression is that some people use Family Echo for much larger family trees than mine. For instance, I have some great great great great great great great great great great great great great great great great grandparents and some 5th cousins. But I don't have everything built out to that distance. The FAQ does say:
Q: Why can't I add more distant relatives?
There is a limit on which relatives can be included in a tree, based on their distance from the tree's founder. This limit helps ensure privacy for family members, and prevents the tree from growing indefinitely. If you reach the limit, click the 'Create new family' button to start a new family branch from the selected person.
