Often it’s best to get two to cover a whole cage. For example for a 5x2 c&c cage you wouldnt want a liner of that size as the washing machine wouldn’t cope, so you’d get a 2x2 and a 3x2 and use them both in the cage at the same time. (So you’d obviously need multiple of them so you’ve got enough to change every few days when one is in the wash).
You won’t want to cut a pre-made cage liner to size as all the stitching will end up being cut off and the three layers would fall apart.
Pee pads are the same three layers but usually much smaller (something like 6 or 9 inches by 6 or 9 inches inches) and are used at the same time as a cage liner. They lay on top of the cage liner so you can replace them daily and not have to change the whole cage liner. They are used under hides and high traffic areas.
The alternative is to use a single fleece blanket which would probably cover the whole cage. If it is too big then you can just tuck it underneath. In that case you need to put in a separate absorbent layer. Something like putting puppy pads would be put down in the cage base and then laying a single layer fleece blanket on top.
A Detailed Guide For Fleece Bedding
Members recommended UK and US sites for fleece cage liners and accessories