Since installing win 7, the screen resolution cannot be adjusted to full size

From which OS to which OS, and on which PC Model is this?
Suggestion is to download the latest driver from the manufacturer website for the correct OS you have just installed.
If else, goto you Display Resolution (On Windows 7 it right click on your Desktop and selecting Resolution) and adjust from there.

In Windows XP:
Start / Control panel / Display/ Adjust screen resolution and settings
In Vista:
Right click on desktop / Personalise /Display settings/ change display size and resolution

Not sure about Win 7 as I do not use it, but I'd think it similar to Vista

You need to set your PC video resolution to the native resolution of this monitor: 1280 x 1024 at 60Hz

Instruction (depend on OS you are running)
Resolution adjustment:
From Microsoft: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/display_change_screen_resolution.mspx?mfr=true

and here:
http://www.helpwithpcs.com/tipsandtricks/changing-screen-resolution-win-xp.htm

and here:
http://www.videojug.com/film/how-to-change-screen-resolution-in-windows-xp-and-windoiws-7

You should adjust the video resoltuion (at the PC) to the native resolution of this monitor, which is 1680 x 1050 @60Hz, the resolution is controlled by the PC, not at the monitor.

Resolution adjustment:
From Microsoft: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/display_change_screen_resolution.mspx?mfr=true

and here:
http://www.helpwithpcs.com/tipsandtricks/changing-screen-resolution-win-xp.htm

and here:
http://www.videojug.com/film/how-to-change-screen-resolution-in-windows-xp-and-windoiws-7

Please leave rating if it helps.

Basic LCD monitor and LCD TV troubleshooting guide:

http://www.fixya.com/support/r6150077-basic_lcd_monitors_troubleshooting

http://www.fixya.com/support/r5093881-lcd_flat_panel_tv_troubleshooting_guide

Failed TV, Monitors, Caps : http://s807.photobucket.com/albums/yy352/budm/

Learn about bad caps: http://en.wikipedia.org/wiki/Capacitor_plague

Go to start, "click" next! click on Control panel, Double click on display, Click on settings "upper right" Lower left you will see screen resolution "click on arrow and move it to the right" Windows will ask you do you want to save these settings "click yes" that will reduce the screen size

As for your 15" LCD BENQ Monitor, "1024 x 768" pixels is the Maximum Resolution
If you increase your screen higher than the Maximum resolution you get a black screen! because the monitor cannot support the resolution higher than "1024x768" pixels.

How to change your screen resolution Win 7-> Link

FYI
Here are typical resolutions for some popular screen sizes
19-inch screen (standard ratio): 1280 x 1024 pixels
20-inch screen (standard ratio): 1600 x 1200 pixels
22-inch screen (widescreen): 1680 x 1050 pixels
24-inch screen (widescreen): 1900 x 1200 pixels

Hope it helps.

You need to set your PC video resolution to the native resolution of this mointor: 1280 x 1024 @60Hz

Resolution adjustment:
From Microsoft: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/display_change_screen_resolution.mspx?mfr=true

and here:
http://www.helpwithpcs.com/tipsandtricks/changing-screen-resolution-win-xp.htm

and here:
http://www.videojug.com/film/how-to-change-screen-resolution-in-windows-xp-and-windoiws-7

Try adjusting the Screen Resolution to a different size. Based on your operating system you can right-click in the center of your desktop and select either (xP) Properties then the Settings tab or (Vista/7) Screen Resolution and under the Resolution area try adjusting the slider and click Apply until you get one that fills in your complete LCD.

There is a micro cable that runs down one side of the screen then around the hinge then under the bezel and connects to the motherboard.
If everything is fine there then follow the process mentioned below.
right click on the desktop
click on "properties"
click on "settings" tab
click on "advanced options"
click on the "monitor" tab
uncheck the box that says "hide the modes that this monitor cannot display"
if you now click the "adpater" tab and click on the button to "show all modes" a full list of all screen sizes should appear.
click "ok" to close the advanced options window
you should now be looking at the screen setting window where you can adjust the screen settings by sliding the screen resolution tab left or right to the desired scree size for installing the program.

Your problem is that the correct video driver is not installed and therefore cannot use all the video RAM.(the XP version of the video driver won't work with Win98).
When you adjust to True color at a higher resolution it drops to a lower resolution because it has insufficent video RAM, but you will get a higher screen resolution at a 16 bit color because it requires less memory (video RAM).
Install the Win98 driver for the graphics card and then it can use the full video RAM at a higher screen resolution with 32 bit color.