Determining the best mailbox quota size for your users

to 100 MB if they're not heavy e-mail users.

If you give your end users a larger mailbox today, it'll be pretty well impossible to reduce the limit in future. Keep tight control over mailbox quotas and only increase them if demand warrants, and only for specific users. Otherwise you risk having massive database bloat (and associated storage costs) due to the many "packrats" in your environment.

The one comment I would like to make is these quotas force PST usage or make people sign up for free Google or Yahoo 1 GB mailboxes. Disk space is cheap and expandable in today's world. With e-mail being mission-critical and mobility devices on a massive rise, this answer isn't realistic. (If you want to get into compliancy, ownership and archiving, that's a whole new ball of wax.) We are a service and if we don't meet our customer's needs, someone else will. That why IT departments crumble; they forget their place in the business model.
—Poomba1