You can change the field width using VBA. The property you are looking for is Width.
However, I suspect something else is going on. You should be able to save the field width when you set it manually.
Do you have the form's "Allow Design Changes" property set to "All Views" or "Design View Only"?
If you are allowing design changes only in design view, then when you manually set the width of the field, changes won't be saved.
Try working with the subform directly (don't use the main form). In design view make sure that the you allow changes in all views. Show data in form. Adjust the width of your field, save changes and close the form. Then open the main form and see if the width isn't as wide as you previously made it.
Randall J Weers
Membership Vice President
Pacific NorthWest Access Developers Group