You don't. JSP has no way of knowing anything about screen sizes or other client-side information. You will need to perform some client-side operation to find out this kind of info. I do not know what the PDA client allows, but if it is Javascript-capable,
you should be able to do this with some simple Javascript code. If not, then you need to find out what the PDA client can do for you in this regard.
I'll move this off to the
HTML/Javascript forum for any further discussion.