If the videos or any content you want to add to your post / blog are to large. You can go into the HTML code and find were the item is and change its size.
for example, the video on side of my blog (I added it as a gadget using HTML upload function)
if you change the # 225 or 149 you can resize it to fit. Just make sure you subtract or add size evenly for both numbers to keep item it scaled. So if minus 100... one side would say 125 other would say 49
<iframe width="225" height="149" src="http://www.youtube.com/embed/0TPbRRjpsIU" frameborder="0" allowfullscreen></iframe>