FileMaker won't allow you to make the Body Part smaller than the edges of all objects, this includes labels, fields, portals, etc.
Open your layout in Layout Mode. Do a Select All. Look all over for some object's handles (these are the little black squares at the four corners of an object) that are up against the body part where you are trying to make it smaller. You will have to resize or move the offending object to get it away from the Body part edge, then you should be to resize your Body part.