A repair install won't change that setting, but it will reset a lot of other things that you probably don't want to change. If you aren't getting the double arrow with the cursor on the top edge of the Task Bar, right-click on an empty part of the Task Bar and look for an item labeled "Lock the Taskbar" in the menu. If there is a check mark next to that item, left-click on it to remove the check mark and then try to resize the Task Bar again.