there is a standard, but much like everything else, standards change. it's called "single din" and it was very easy to add aftermarket head units. If I recall, it was the 1996 model ford taurus that went to the double din standard (2x's bigger slot) with their integrated control panel which combined radio and hvac controls. That's where we are now, most cars will accept the double din standard, but even that is changing. people want bigger screens, more screens, etc. It won't be long before traditional drop in electronics are eschewed for complete touchscreen control centers. we already have the technology (think ipad instead of buttons & switches), it just takes one manufacturer to make the leap and the rest will follow. at that point adding hd,sat,etc as an aftermarket accessory will have to be in under dash kits and modules.
one more thing. in most of your newer cars, the radio is an integral part of the "system". The onboard processor's are programmed to recognize when something is not there, and can shut down related systems, or give false dashboard warnings. before swapping your head unit out, consult with your audio installers, and DO YOUR RESEARCH online in the forums that deal with your particular make & model.
I have no axe to grind one way or the other with anyone on this board. Look forward to the continued lively discussions. Thank you.