Uhhh...I would ask exactly what a sand dollar sized fish is...Sand dollars are living animals that grow. I've found them as small as a dime and as big as a breakfast plate (sorry about more general size references...ha ha). Snake is right, though, angelfish size usually refers to the body size. I would look at your cost differences between the half dollar and silver dollar sized angels. If you think the price is neglegable between the two, go with the larger fish. I would ask that they pack the fish extra light. I think you'll find it easier to pay more for shipping and have fish arrive alive than to skimp on shipping and go through the hassle of a D.O.A claim.
I know this if off-topic, but other size references used in the fish trade as follows:
golf ball
bigger than a golf ball
baseball sized
'bout the same size as the top of a Coke can
tea saucer