Could be a few things - is the screen maximized? See the top right corner of the screen where you normally click the [ X ] to completely close the window? Next to that to the left is the maximize button [ [ ] ] and by clicking it toggles back and forth.
Screen resolution? - in XP and from the desktop, right mouse click, choose properties, click on the settings tab, move the setting slider to the right one notch, click yes then apply. This will make all icons smaller giving more space on the desktop for more icons, the down side is everything is smaller, but more resolution and more on the screen at the given time. - in windows Vista op system, instead of properties after you right click, there should be an option for "Display Settings". It's basically the same as XP, just a different term. If you don't like the smaller icons, you can always do the same except move the slider back to where it was before you made any changes. Note:.... just move the slider when first testing to the right only one notch. Unless you have a high end video card installed, then your limited and moving to the right to far over tax the on-board video and the system becomes unstable.
One you get your settings, if this works for you, properly shutting down enables the system to save your settings.
Now I'm not 99.999% sure if your talking screen size or the task bar at the very bottom of the screen. If you want to do things to the actual task bar, you can put the cursor on the task bar, right click, chose properties and there are options for that.
