I'll assume that you are using Internet Explorer. If that is the case, you can fix that by doing the following:
- Close all browser windows but one.
- Open a new window from a link on the remaining window.
- Close the old window (not the new one that just opened up).
- Adjust the window to be the size you want all the new windows to be (you cannot use the maximize button for this, you have to actually change the size of the window to be what you want the windows to automatically open up as).
- Hold down the Ctrl key while closing the window.
From now on, all your new windows should open up to that size until perform a similar process telling IE that you want all new windows to be the new size.
Most likely what occured is that you manually sized a window to that size, closed it, and IE remembered that as your preferred window size.