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Planets are small enough and far enough away that they will never fill a significant fraction of your field of view even at you scope's highest usable magnification.

If you want to see a larger disk, you need to use a higher power eyepiece. Remember that the smaller the focal length of the eyepiece you use, the higher the magnification you will see in the scope.

For example many of Celestron’s basic telescopes come with a 10mm eyepiece as the shortest focal length in the box with the new scope. Using a 5mm eyepiece instead will double the magnification compared with the 10mm eyepiece. Your scope will show a larger disk than what you are seeing now. It will still be a small disk, but you will potentially be able to see more detail than with the provided eyepieces. 

However, that does not guarantee a sharp view because of the earth's atmosphere and seeing effects. For more on this topic see the article below:

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2481