If they are in the primary filegroup, new data will be automatically written to them. SQL will not however re-balance existing data. Read up on proportional fill for how the data will be distributed.
As for how big they are, well, how big do they need to be for the future growth of your database?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability