After you've opened the table you want to modify in Design view, click the number field whose size you want to change to select it.
The lower half of the table window shows the properties of the selected field, the first of which for number fields is the Field Size. Click the Field Size text box.
Click the down arrow that appears in the Field Size text box and select a field size from the list that appears.
Click the Save button on the Standard toolbar in the main Access window to save your changes to the table.
As with text fields, you can also change the size of number fields. Unlike text fields, however, the size of a number field is not tied to the number of characters that field can contain. Instead, Access uses a special name for each available field size. The most common is Long Integer, which is the longest size for a number. (You can store numbers between ?2,147,483,648 and 2,147,483,647.)
Understanding Number Field Sizes
The most common field sizes for numbers are Double and Long Integer. You can, however, use other sizes for special purposes. Consult the online help for detailed instructions on the other sizes and their uses.