If it is too big, then that is a change of mind and you are responsible for paying the postage.
As long as you have opened a case through Ebay, you can arrange to return the TV through Parcelforce.
If you can't manually update the tracking in the case, then you can contact Ebay Customer Services by getting a callback and ask them to update the tracking for you.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)