When creating a surrogate key field, is there a recommended "default" size for the number? That is, is it most common to take the default Number size of 9 digits (I think that's the default), or should I carefully restrict the size of this field?

Do larger number fields take more space in the database? EG does a Number(9) field storing a value of 3 take up more space than a Number(4) field? If so, is it enough of a difference to be concerned about?