Board size is not available as a variable in WRDS databases (you can find it in Datastream – it’s an item from Asset4 (Mnemonic CGBSDP060)).
This means that when you use for example RiskMetrics, you have to calculate the board size, by counting the board members.
You can do this in Excel by making a new variable, by combining the year and an identifier of the company for each person in your data set (for example taken from RiskMetrics). Then you can count how often that combination of year and company exists in the dataset, using the countif-function in Excel. This will give you the size of the board.
Of course this only works when you haven’t used any conditional statements to select or delete specific board members!