editing mobile page in Webmatrix

hi everybody, 
im new with webmatrix and a beginner in site building.
i just have a question about how to edit the size of the text box on my mobile version of my site i made in webmatrix 2
as you can see, i made an screenhot an cut out the window of the simulator:
but you can also go to the page yourself.. www.jvmetaal.nl
its allready online :)

a
as you can see on the picture above, and also when you visit my site www.jvmetaal.nl with your mobile phone, go to the order page, and you will see that the space too fill in your request is really too small..
so please can someone tell me where to find the code line to change the size?

thanks,
Jeroen