Best settings for xMAME

Hi all Amigans
I have putted some screenmodes in Devs/Monitors/Radeon like
384x224 and the likes and want to know why doesn't show well as it seems that the new screen resolutions are bigger
than my monitor surface.It happens with the 384x224 and others so I deleted them from the Radeon ToolTypes and Picasso96 too.
What can I do to introduce new screenmodes that fulfill my
monitor's screen and does not go out of the size of the screen?
May I configure Picasso96?
Anybody have similar problems to scale this screenmodes?
It's so important cause with the exact screenmodes on hand
MAME gains speed and the visuals are spectacular when the
screen is well suited.
Help is really appreciated.