dont know whats the perfect size?

Double din unit is the size. 

why does it say certain head units are compatible with certain cars? if they are all universal?

They aren’t all universal. 

 

On 24/07/2019 at 21:40, Khalil said:

why does it say certain head units are compatible with certain cars? if they are all universal?

 

The only thing universal is the DIN size. If the unit has the wrong connectors, then you won't be able to hook the unit to power or speakers.  This is why it says it is only compatible for certain cars that have the right connectors.