Pick the kind of image you want to produce and size it.
Heads up: elemental customize only emits a raw disk image or an ISO — there is no --type qcow2. The qcow2 conversion and resize to the size above only happens when this wizard builds the image for you. If you go the download route you'll get a raw image and need to qemu-img convert -f raw -O qcow2 yourself.
elemental customize
--type qcow2
qemu-img convert -f raw -O qcow2
/dev/vda
/dev/sda
/dev/nvme0n1