I have heard that as the size of murti grows it attracts more divine power and requires more Pooja. Hence one should not keep a large murti at home but how about small metal one around 3-4 inch. Can I keep a small idol?
Rather than the size, please focus on the arrangement of deities. There is nothing wrong in keeping a large idol of Hanumanji if you have a big house!
Couple of sites for your reference: