Size will matter on Melbourne's busiest train line with platforms to be extended at 13 stations as part of a $660 million upgrade.

The longer platforms will cater for the new high-capacity metro trains that will first travel on the Cranbourne-Pakenham line. Work will begin to extend the platforms later this year.

In total, 18 stations on the line will have the longer platforms, including five stations being rebuilt as part of the level crossing removal program.

The new trains, which will be 20 per cent longer than the existing fleet, are currently being built.