planetary gear box dimensions???

Do you just want to "draw a planetary gearbox" or you'll use it in some project of yours?

For a project, not any gearbox will help; you'll need one with specific gears in specific dimensions...
For anything else, just search the internet...

Some dimensions are determined from the specifications. From input speed, reduction and power to be transmitted, will either enable us to select a gear unit from a maufacture's catalog and most likely be able to download a CAD model, or if we are designing our own unit, we can use this specification to calculate input / output shaft sizes, and a suitable gear module. Once we know space required for gears, work out bearings etc, we should have some sizes we can work to .

I suggest you to take a look at the Machine Design book by Robert Norton, at page 653. He shows some nice details on how to understand the planetary gearbox anatomy.
I attached my Excel sheet that calculate the gears (number of teeth - metrical) for the planetary gearboxes I make. Feel free to use it. Hope it helps.