Under the File menu go to Preferences -> units and select "inches" in the box below that enter # of decimals. Click "Apply and "OK". After you create a standard you need to save it under a new name, you cannot delete or change an existing standard. Now you need to select your new standard in the Menu Dimensions -> Standards. Here you will see your new standard and the Stacked Fraction and Fraction standard should you ever want to use one of those.
Under the Dimensions drop down select "linear" and you will see a selection of decimals to display. The number of decimals that you entered in the "Units" box has priority over this setting. If you entered 3 in the "Units" box and 4 in the "Linear" selection won't get more than three places.
When you enter dimensions there is a "Text" box in the lower left corner. There is a # sign in this box, any text entered before the # sign is displayed in front of the dimension and any text after the # sign appears after the dimension. Delete the # sign and no dimension will display but any text entered still does. You can also edit this in the "Edit Object" palette. Window menu "Edit Object".