Originally Posted by dabac

You want wheels with a 20” (451 mm) diameter.
And you probably want to check the type of hub.
BMXes can have both freewheel and cassette hubs.
Cassette hubs use a smaller rear sprocket and may not work that well with a BMX set up for a freewheel hub.
Wheels may be sold w/o sprocket/freewheel, so you might need to buy the right tool to transfer yours.