Sure. To edit a stencil:
1.Click the stencil's icon (in its upper left corner) and choose "Edit" to
enable editing.
2. Then right click on the shape (the Entity shape in your case), and
choose "Edit Master"
3. Visio will display the master shape in a drawing window. You can edit
the shape as you would any visio drawing. In your case, select the shape
and decrease its font size.)
4. After making your edits, close the window, Visio will ask you if you
want to update the master (i.e. Update 'Entity'? in your case.), choose
"Yes"
5. Click on the stencils icon again (upper left corner), and this time
choose "Save"
6. Finally click on the stencil's icon one more time and this time choose
"Edit" to disable editing.
One gotcha to watch out for: The database modeling solution has two
stencils: the "Entity Relationship" stencils and the "Object Relational"
stencil, and both of these have a copy of the Entity master shape on them,
so you may want to edit both copies.