I assume you are talking about an internet link for your DC based applications. Sizing the link will vary alot based on how many devices, applications, users and other workloads you have in the DC along with their outside communications needs.
Is this a move, migration, upgrade or a brand new build?
What you need to make the determination is some data to generate a reasonable estimate.
As an ISP we provide services as small as 5 meg to multiple 10 gig based on customer needs.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home