Windows scaling format

[Mr.Bill]

Hello,

I have a little problem. I would like to scale up to about a Windows Comand an image based on its format.
Size: Landscape: max. 640 px width, max. 480 px height
Portrait: max. 480 px width, max. 640 px height

How can I write to a single converter command? It needs to run on Windows and may be used only with a command.

Thank you very much

[Bonzo]

I do not think that will easily work in a comand unless you call a batch file.