Add the ability to stretch a field in a custom form

Description -

Why is this feature important to you - When creating a field in a custom form and viewing item in Preview mode, items are truncated.   Would like the ability to stretch the field box in order to see everything.  

How would you like the feature to work - Be able to stretch the box within the window so items can be viewed

Current Behaviour - Field box is not very big, so you can't view everything.  Items are cut off.