Before you define ideal size for your community, you need to define what is "successful community".
There can be different measures of success.
One may consider communities as successful as long as they are meeting member's objectives of connecting. Then size may not matter as long as the member of the community are committed.
Others may consider communities to be successful based on the activity, # of docs, posts, etc. Then also it is the size of active members rather than total members that matters.
So, define the objective of the community and how you define successful before trying to define ideal size.