I've worked with Salesforce since 2001 in companies ranging from 4 to hundreds of sales people and the system has been able to scale and perhaps more importantly expand (functionally) to the needs of larger organizations. In fact in my previous company, as a provider of a solution that integrated with Salesforce, we had customers that had thousands of sales people using Salesforce.
Salesforce is a web-based CRM application that enables users to forecast revenues and track leads. It is a leading sales, service, and marketing app. Salesforce.com's Sales (CRM), Service, Platform and Marketing applications are designed to help companies connect with customers, partners, and employees in entirely new ways. According to the vendor, on average, reps using Salesforce see: a +29% increase in sales from greater visibility, +34% increase in sales productivity, and +42% increase in forecast accuracy.