Question

why solids have a definite size and shape​

Answer 1

Because atoms are packed tightly.

Or strong intermolecular force.

Answer 2

solids have a definite size and shape because the molecules of solid are in fixed position and close together . Although the molecules vibrate, they cannot move from one part of the solid to another part. As a result solid cannot change it's shape and size.

Hope this answer help you.