How do you measure your pool to know what size liner you need for an above ground oval pool?

It all depends on what shape the pool is.
Rectangle : L X W X Height
Free-form pools like kidneys etc, need a lot more detailing.

L plus W plus H

The way you would do this is by measuring from the inside on the wall, to the opposite side, inside on the wall. Do this in both direction on the pool. Most people measure from top rail to top rail, this gives them a wrong measurement. Let's say you have a 15x30 oval, you would measure from inside to inside, on the wall to get you measurements. To determine the height of above ground pools, you will measure from the bottom track to the top of the wall. Let's say it is 52"
so you would have a 15x30x52 pool. Just a little side note, not every above ground pool is exactly a perfect measurement, saying that a 15x30 might read 14'8"x30'4" but in turn it will be a 15x30 foot pool.