How do I increase the 'Products Model' field length

[Gnarlyman]

Hi, this is my first post and I have managed to work most things out for my shop myself or off the helpful posts in the forum, but this one thing is driving me mad.... How can i make the 'Product Model' field accept more than 12 characters? If I can get this sorted it will stop me having to bother to work out how to add in more fields and then get the search to work across extra fields.

 

Thanks in advance.