According to Wikipedia the current cars are 48 feet long, and most of the stations were recently (or are in the process of being) rebuilt to accomodate 6-car trainsets. Older cars were 48' 10" long, but the article said most of these cars are out of service except a small number saved as "backups" in case the newer cars need to be taken OOS.
Not sure if 48 feet includes the coupling between cars or not, but multiply by 6 and you've got yourself a rough estimate on how long a platform needs to be. Might be able to find plans (or at least some basic info) on the manufacturer's website that might give more precise measurements and/or platform heights.
Another way to do it, if you're up for some fun with Photoshop and math skills, is to check out a photo of the front or side of a current car. The more "straight-on" the photo is, the better. Measure how many pixels wide or long the car is. Then measure how many pixels from the ground (or rail?) up to door level.
Knowing the cars are 48 feet long and 9-foot-3-inches wide, you can convert pixels back to "real life" measurements to figure out how high off the ground those platforms should be.