Well, a lot of folks consider SMB as one cookie but there really two scales involved: Small and Medium! According to Wikipedia, the nomenclature based upon number of employees is as below:
I find the Australian/European scale of less than 50 for small business a much practical number as compared to 250 for the US which kind of falls under a medium sized business. For the purposes of capacity planning, we can say:
Minute/Micro Business : 1 – 10 Employees
Small Business: 11 – 50 Employees
Medium Business: 51 – 250 Employees
Large Business (Enterprise with multiple branch offices): more than 250 Employees
That being said, a good plan and design should always consider scaling from lower to the higher bracket. Every business wants to to grow and it would be bad engineering if the IT/Infrastructure investments were not scalable and sustainable as the business scales! Scalability is one of the prime challenges of a good design.