fat quarters are 1/4 yard cut 18" along the selvage and 22" (or half of the width) ... I usually cut mine 18" - 20" by 20" - 22" depending on the fabric. I cut a half yard then fold selvages together and cut along the fold for FQ swaps. hope that helps ....
most of the time when a pattern calls for FQs, they must be cut to specific sizes, so you don't have to start with FQ just at fabric as big as FQ .....