It is typically 100 s of Gigabytes to Terabytes. A Data Mart is a Single Subject Data Warehouse and it is often Departmental or Line of Business oriented. Also the size of a Data Mart is typically smaller than a Data Warehouse mostly less than 100 Gigabytes. Data marts can be delivered in a matter of months, and for hundreds of thousands, rather than millions of dollars. That defines them as within the range of divisional or departmental budgets, rather than as projects needing enterprise level funding.
Data marts are easier to get through politically for at least three reasons.
First, because they cost less, and often don't require digging into organization-level budgets, they are less likely to lead to interdepartmental conflicts. Second, because they are completed quickly, they can quickly produce models of success and corporate constituencies that will look favorably on data mart applications in general. Third, because they perform specific functions for a division or department that are part of that unit's generally recognized corporate or organizational responsibility, political justification of a data mart is relatively clean. After all, it is self-evident that managers should have the best decision support they can get provided costs are affordable for their business unit, and the technology appears up to the job. So, data marts have become a popular alternative to data warehouses.