variable space

I don't understand how much space variable will occupy depend on what factor ?

These are my doubts

Does only compiler decide how much space variable will occupy ?

Does only target machine decide how much space variable will occupy ?

Does both compiler and target machine decide how much space variable will occupy ?

I think compiler decide how much space variable for specific data type

The compiler writer looks at what the machine is capable of, then decides on appropriate sizes for things like integers, chars etc.

You as a user of that compiler are stuck with those choices.