Size effect, as its name indicates, takes the size of a firm as a relevent variable for equity valuation.

If you are planning on empirical works, the size effect should be defined as the market value of the firm in question,

which can be calculated by multiplying number of ordinary shares outstanding by the price of shares.

公司的规模会影响到股票的收益

具体问题具体考虑,一般SIZE 可以用 销售收入,利润,市值,公司员工数量等等来定义