As you don't say what operating system, graphics adaptor or driver version you are using, or what make and model your TV is, it is impossible to say with any precision - and I, for one, would probably not be familiar with most of the variations...
However, in general, I would be inclined to Right Click on the Desktop, select 'Display Properties' or 'Customise' or whatever you can find, and look at all the options and sub-menus - there may be something in there that will allow you to do it.
It may be a question of altering the Resolution of the Auxiliary or 'B' display output to match the resolution of your TV - say 1920 x 1080 or whatever. That's assuming that the computer graphics adaptor can produce a sufficiently high resolution.
Alternatively, you may come across a setting that allows you to expand the active part of the video output to fill the screen.
Lastly, have a look at the instruction manual for the TV (if you've got one that's actually written in a language that you speak - always an issue here ) - and see if there are options within the TV setup menu for adjusting the display size - some of my LCD TVs have the facility, and some don't.
I did have one combination of laptop and TV where it seemed impossible to get the settings on both items to give other than a small 'window' to peer through... I gave up and started using Android boxes -
much more satisfactory!
Paul Aldred markets a variety of
boxes specifically set up for viewing UK (and other) TV programmes, and I bought one of his 'basic' models just to try it out, and have found that it compares favourably with ones that I've paid more than twice as much for - and it's simple to use! I find that Multimax give me a (generally ) stable enough internet connection to work fine with all of the boxes, but I'd recommend using a voltage stabiliser to protect the TV and anything plugged into it - the voltage surges that often occur after power cuts can put paid to all of your telly viewing!