You should put a fair amount of thought into designing your SCOM infrastructure. You need to make sure not only that the design works efficiently for the existing requirements you have, but also that you will be able to accommodate the future growth and expand your current Management Group without compromising its efficiency or performance.
SCOM includes the option to autogrow your Operations database. You can enable this to allow the DB to grow in size along with your overall SCOM environment. In my opinion, you should not do so, however, unless you are absolutely forced to.