Accurately identifying and sizing your target market is one of the most difficult exercises that your company will face. Factors to consider include general economic conditions, sector growth, market penetration, volume, and price. Markets should always be defined using both "top-down relevance" and "bottom-up realism."
Do not overstate the size of your market.
Never say that the market for your product or service is huge, and if you just capture X% you will be a $X billion company.
Explain why the market is large and describe how it is segmented. Clearly identify and quantify your addressable part of that market.