SMB is an acronym that refers to the size of a business. Classified as Small and Midsize Business or Small to Medium Business, this category is typically measured in number of employees and annual revenue. While the numbers vary a bit based on who is defining the SMB, Small Business will have up to 100 employees and annual revenue of less than $50 million. Midsize Business will have from 100 to 999 employees and annual revenue between $50 million and $1 billion.