@aexnitin No, don't use Codecademy Labs. They're not being maintained anymore, and will likely be removed soon. Use https://repl.it or http://codepen.io instead. Repl.it is built by the same guy who made Codecademy Labs, but has more languages and is being maintained. And CodePen is one of the best places on the web to mess around with webpage-y stuff.
I think that's already because you're on the index.html tab then... ?
The index.html tab is where you put your code. The result tab is where you view the result of your code, and it's to the right of the code editor. To view the result full size, click either the View in full screen button on the bottom of the preview window, or the resize icon in the upper-right of the preview window.