I think that this is a problem in Odoo V9. I have just recreated it on Runbot.
When the User has Properties on Lines checked the form view is used when editing sales order lines.
If the user creates a new sale order either from the Sales Orders menu, or via a customer record and clicking the $Sales button then the Product field size is a sensible size, as it was in V8.
The problem occurs when the user creates a new sale from the Quotations Menu, or opens an existing quotation from the Quotations menu, on editing the sale order line the form view gives a tiny field for the Product. Once the Quotation becomes a Confirmed Sale and is accessed from the Sales Orders menu the field becomes a sensible size again.
Does anyone know of a reason why this happens? or how to overrule it?
UPDATE - 29/2/2016
https://github.com/odoo/odoo/issues/11081#issuecomment-190275661
See github link above.
you can add colspan to render field input below the label... The half width, is because you can have other field like according to your configuration: http://www.awesomescreenshot.com/image/990464/6971ee1a6f77d46a3cb24afbf175d5ea.
Please could you give me an example of the xml required?