I don't think CRT televisions have an easy way to turn off overscan. By their nature, they are designed to cut off some of the picture because it will never exactly line up with the edges of the tube.
If you can find the code to enter service mode, you may be able to adjust the picture geometry manually, but do so at your own risk.
TiVo Mini X 3
Left DirecTV 4/15/07. Left TWC 8/8/11. Now on Comcast Xfinity.