Smart Sheets are made for a US and Canadian standard size crib mattress (27.5 x 52 in / 70 x 132 cm). Your baby does not need to be sleeping in the crib in order for you to measure them. If your baby is still sleeping somewhere other than their crib (i.e., a bassinet), you can still measure them in the crib by putting them on the Smart Sheet while it is on the crib mattress.
The American Pediatric Association recommends that a baby will be sleeping Alone, on their Back in a Crib (“ABC”). You can read more about safe sleep practices here.