The picture control is pretty basic, just allowing the attachment and display of an image. Hopefully the intent of the field is known for each form, maybe it is a photo, or a drawing that is typically pretty consistent. That way the picture control can be sized appropriately. If there are some various uses and need for a couple different sizes you could have multiple controls, bound to the same field that use conditional formating to hide/display as necessary or have a drop down to change them to the appropriate one. Otherwise I do not believe there is any way for the user to dynamically alter the sizing of the control. The user would be responsible for editing the picture before inserting it into the form.