I believe you would do this by specifying the value in the database field itself. For example if your field was named decimal_value and you were using MySQL you would specify decimal_value(x,y) where x is the total number of digits you wish to display in the value and y is the number of digits to the right of the decimal. I think you can do this on the Datasource Tables tab by setting the field type to decimal, the size colum to the max number of digits to display, and the scale column to the number of places to the right of the decimal value. Hope that helps.