Thank you for your post.
No, it is not possible to set the field size from a script. One possible solution is to have two different layouts with the two different field sizes. You can then check the screen width and select the appropriate layout. For example:
If [ Get ( ScreenWidth ) > 768 ]
Go to Layout [ <expanded layout> ]
Go to Layout [ <normal layout> ]
Let me know if you need clarification.
I found your reply a great help, this makes me create a layout for widescreen users and one for normal screens. I have programmed the whole database in with 1024 x768px.