Hi Rachna! This is from the article above:
Appropriate Recording Size
If I want my screen recording to fit just right on my Storyline slide, I need to set my recording area to be the same dimensions as my Storyline slide. (Psst: You can update the story size by going to the design tab of the Storyline ribbon and clicking the Story Size button.) But here’s a pro trick: if you set the player to fill the browser window (as I do in the example here) and you want your screen recording to remain nice and crisp, set your recording area to be at least 2x the side of your story slide. So, for example, if my story slide is 720 x 540, I set my recording size to at least 1440 x 1040. This way, I can enlarge my course without having the recording become fuzzy or pixelated.
So please make sure you're using the same aspect ratio for your recording and your slide size, and try setting your screen recording area to be bigger than the slide so it always remains a nice, crisp background.
I hope this is helpful and answers your question! :)