Changing the resolution at start-up time:
There are two sides to this; the framebuffer (for text consoles) and the X server.
To change the framebuffer's resolution, you need to enter the command-line parameter
you will then be prompted to enter a scan-code when you boot up, with a list of resolutions (in characters) that they correspond to.
To change the start-up value of X, edit the file /etc/X11/XF86Config, and change the “Modes” setting to the resolution you want in pixels.
There are also utilities to change the resolution of the terminal within a program, e.g. svgalib has routines to do this, so there's probably commands that will work on-the-fly as well.