1) If I use Oracle sqldeveloper as a migration tool, does it support any platform for Oracle DB as a target.
=> yes it does.
2) With Oracle sqldeveloper can we migrate large DB's. Any Benchmark time for large DB's say 200-300G.
Yes you can. SQl Developer allows you to migrate in an ONLINE and OFFLINE mode the data. For a large amount of data the online method is a worse option to choose. You'd better use the offline data move based on SQL*Loader which you can then run on the database server directly w/o any network delay and you can also run it in parallel. Unfortunately I'm not aware of any benchmarks, but it is always a good idea to check out http://www.oracle.com/technetwork/database/migration/index-084442.html whcih provides a lot of details and migration white papers.
3) Any other method which can be used to Migrate
=> use GoldenGate which allows you to replicate data between Oracle and SQL Server.