A jelly roll will have strips that are 2.5" wide by WOF (width of fabric, which is usually 44-45").
A fat quarter measures 18"x 22" instead of a regular quarter (9xWOF) because it's often more usuable that way.
Some shops charge more for fat quarters. My favorite local shop does not. But my favorite "small cut" to ask for is 1/3 yard.