You can predefine the size of a subform before adding it an input form. Create a subform object in the input form beforehand and drop the list form on it. Detailed instructions are included below.

1. In Design mode, open an input form in the Form editor.
2. Create a subform object on the input form.
3. Resize the subform object.
4. Open Explorer and select the Forms tab.
5. Drag a list from from Explorer and drop it onto the subform obect.

On the input form, the list form is limited to the size of the subform object.