The sizes should be in proportion to each other.
So if 6378 km (i.e 637800000 cm) radius Earth is taken as 11cm radius football,
then the size of the moon model, let it be x, should be in same proportion.
∴x= 11/637800000 * 173800000= 2.9974913766070868610849796174349= 3cm
So size of moon should be approximately taken as 3cm