Form fields are cut off when site is viewed on tablet or phone

Hi all

The contact form looks great on a pc, but when viewed on a 8.3″ tablet or 4.5″ phone, the following fields are “cut short” (to the right-there is no verticle line at the end of the box)

Name
Email
Subject

The only field which looks good on all devices is the message field, I suppose it is because it adapts to the different screen sizes, due to the little clickable corner on the bottom right hand side.

Any way I could make the afore mentioned fields “resposive” as well?