Keyboard mapping and screen size

I have a few questions about tn5250:

(I'm running Red Hat 7.2 if it makes any difference)

1.  When I start a session (by typing "tn5250 myAS400") it only occupies a
small portion of the screen.  My monitor is set to 1024 x 768 and the
session seems to be defaulting to 640 x 480.  How do I make it use the
entire screen?

2.  I would really like to re-map some of the keys, for instance I like my
"Enter" key to be Field Exit and I like my "Backspace" key to be the same as
Cursor Left.  I've tried modifying the us5250.map file but the changes don't
have any effect.  How do I change the keyboard mapping to suit my fickle
tastes?

The second question leads me into another one.  I would really like to make
a simple graphical program that would allow users to re-map the keyboard,
much like the way Client Access does it.  It looks like all I would have to
do is write a program (probably in Java since tn5250 is cross-platform) that
would generate a file like us5250.map.  Assuming that tn5250 can be made to
read a file like us5250.map, how can I find out what values are valid to put
in there?  Where can I find out exactly how this file is structured?

I appreciate any help I can get.

Thanks,
Brian