I've done a failover cluster with an instance that had 1 600GB database and 1 1TB database. No storage problems (well, other than a badly designed very slow SAN)
The size of the database isn't a factor in a cluster failover, the storage is shared, the ownership of the storage switches from one server to the other, there's nothing copied.
Could you describe the problems more?
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