Would a small atrial septal defect cause a murmur? or does it usually have to be a certain size to cause murmur?

It can: Murmurs occur when blood flows turbulently from one area to another area. If the pressure between the two atria is different, blood can flow through even a small defect from one side to the other. In fact, small defects often cause louder murmurs because the blood going through a smaller hole is noisier than through a big hole.

ASD: ASD has to have significant flow to cause murmur. Murmur in ASD is due to increased flow across pulmonary valve and not due to flow across asd. Excessive flow through lungs causes damage to pulmonary vascular tree. This will decrease the flow through asd. A murmur can then disappear due to absence or reversal of flow. An ASD must be treated before it is too late!

Possible: A murmur is literally the transmission of vibrations through the body to the skin surface. I mention this because an individual's body type plays a role in murmur quality. Small asds may cause murmurs in thin children whose hearts are close to the chest wall. On the contrary, a large ASD may not create a murmur in a large person, with significant tissue between the heart and chest wall.