Like the above poster said, the key is /etc/X11/XF86Config
Even if you do get the default resolution switched higher it may not work if you hvaen't put in your Horizontal and Vertical sync data into XF86Config.
Hopefully you have the paperwork that came with your monitor. Look in the paperwork for the specifications. Among the seemingly foreign looking stuff you should find specifications such as:
HSync (or Horizontal Synchronization)
VSync (or Vertical Synchronization)
These will usually be expressed as a range. For example, my monitor's (read: MY monitor's..don't just try these values because I type them here) HSync range is: 30-95 (these figures are expressed in kHz)
and my monitor's VSync range is: 50-160 (these figures are express in Hz)
These are critical elements of your XF86Config file which lets Slack know what your monitor is capable of. By default Slack has a very conservative setting for both of these ranges. This is due to the fact that INCORRECT SETTINGS CAN DAMAGE YOUR MONITOR. MOST modern monitors will not allow damage to occur since they will simply shut down if you have out-of-range values...BUT BE WARNED, IT CAN HAPPEN.
So look for your monitor's data and then put it into the XF86Config file. I had the same situation as you (my monitor was not listed in the Slack list of monitors). Just be sure to get the appropriate information in there and you should have no problems.
There is lots of information on this forum (and on the net in general) about getting these specifications into XF86Config and also on setting default and otherwise available resolutions.