Swapping tall narrow radiators for short wider ones- how to calculate

We are doing a renovation and want to swap out some tall radiators for shorter ones to work under a window and not block part of the window itself.
Probably going from a 36" tall and 12" wide radiator to ideally a 20 or 24" tall radiator to sit below the sill.

Assuming the the current tall radiators are sized right, is there a way to figure out what size we could swap them out for that will be shorter (and I assume they'd need to be wider since they're shorter)?

Is it a matter of LxWxH calculation being the same and swapping for a shorter wider radiator?

Thank you for your help!