Not surprising really, 1440 is quite bit. You need to know how to adjust the screen resolution, and you're probably going to have to do this via a text only terminal(?) - there is some reading matter:
have a look at this page -
You exact problem is not discussed, but there are comments on how to adjust X screen setting in slackware which you should be able to adapt for your purposes.
failing this - try:
as the official sites advice.
lastly - a note about questions [set flamers to "light brown"]- do read anything which tells you how to ask questions in this forum. You posted to linux hardware, but did not include any details about your hardware... what is your monitor?
However this is really a slackware configuration question - you have written mostly about config files - so perhaps it should be in the slackware distributions forum? Whatever, it would help to let us know what version of slack you are running, and what the kernel is...
OTOH: you did give a good idea of what you have tried and what you hoped to acheive: things normally left out.
Good start - I've done worse. [flamers off]