A question people ask
Who should we stop selling to?
The customers who cost the most to serve and teach you the least. Deciding who you are not for is half of a market definition.
Most companies have a list of target industries. Very few have a list of customers they will politely decline. Without the second list, the first one doesn't mean much.
Start with margin and cost-to-serve, not size. Some of your largest customers may be your least profitable, and some of your smallest may be the clearest signal of where the market is going.
Saying no is not lost revenue. It is capacity you can point at the customers you actually want.
Read the longer article: How to define your target market in a technical B2B company