What will be the effect on circular coil carrying current in a megnatic field. How will the coil expand or compress???

It will depend upon the direction of current flow in the coil and external magnetic field.
As its not mentioned in the question so let me assume that current flowing in clock wise direction and magnetic field is outward perpendicular to the plane.
Use F = I dl * B and see the direction of force acting at all the points on the coil.
Please find the attachment.