Well I am not sure which version of Linux you are running, but if it is Red Hat, then it probably came with WindowsX. I will assume this is the case. Log into Linux as root. You have to log in as the system administrator in other words. If you do not remeber, then look up logging in for the first time in the manual. It will tell you how to log in as teh administrator. You did it the first time you logged in.
OK. now type startx. This is to start up windowsx. When windowsx loads, it will look a lot like win95, but it will not appear completely the same. There will be some windows open; two if I remeber correctly. One is the shell for typing commands; the other should be the control panel. the control panel should have a display or similar option, and that will allow you to change the desktop size. If the control panel is not open, then look for it in the start menu.