It seems many TV's and computers (be them PC's or lappy's) have problems with overscan when connected via HDMI. If your laptop has a VGA output on it, and you TV has a VGA input, try that as most times VGA to VGA works perfectly - just for some reason HDMI overscans...

Screen size (I pressume you mean resolution on the computer) should be changeable by right clicking the desktop and selecting properties - or going to control panel > display > settings (I don't have Vista so can't tellyou for 100% sure) but this probably won't change your situation - it's the overscan to do with the HDMI transfer thats the problem.

As for screensaver, what player on the lappy are you using? Windows Media Player? If so, go to TOOLS > OPTIONS and click the PLAYER tab, and DE-select (as in un-tick) the ALLOW SCREEN SAVER DURING PLAYBACK option.

If you use VLC media player (if you don't, you should - much MUCH better for video playback than WMP) it should disallow the screen saver during playback by default.