One of the characteristics of air is that to has definite shape and size is this true?

No, this statement is not true. Air does not have a definite shape or size as it takes the shape of its container and expands to fill the space available to it. Air is a gas and therefore does not have a fixed shape or volume.

No, this is not true. Air is a gas and gases do not have a definite shape and size. They take the shape and size of the container they are in.