What are the two formulas in which market size can be measured?

Top-Down Formula
Search all government and trade association figures. Using the Central Statistical Office’s Guide to official statistics (available from Her Majesty’s Stationery Office), the search should begin for government publications which contain the market size data.

Bottom-Up Formula(Demand Side)
Assessing the demand of consumers (or distributors) of a product usually requires an interview programme. Buyers of products seldom publish figures which can be used as a guide to their purchases – this needs to be elicited from an interview.
A bottom-up analysis evaluates where products can be sold, the sales of comparable products, and the slice of current deals you can carve out.