It makes no sense, usually. Well, how larger? The bitmaps are usually too small; sizes of 40x40 or 64x64 can be considered as very big, you hardly can find any bigger. Also, makes sure you have a legal rights to use some available bitmap the way you want.
But the most important factor is this: you can re-sample any bitmap down with acceptable quality, unless extreme cases of a very small ones (16, or 32 pixels are already small), which usually requires manual improvements, which in turn requires special skills and experience. As to the enlargements, all automatic re-sampling methods gives quite lousy quality. (Ever heard of information entropy, http://en.wikipedia.org/wiki/Entropy_%28information_theory%29
For more or less large images (say, some 50 pixels and more), it's way more beneficial to create and store vector image. Each time you need a bitmap, you should scale a vector image to the size of the bitmap (or more, for subsequent re-sampling down later), and export it as a bitmap. The exported bitmaps can be used to create icons as well. One highly recommended product is the open-source Inkscape, please see:
Sorry for misunderstanding. There are more than one bitmap in the icon. Extracting them is easy enough. Please see this CodeProject article:
Extracting and displaying icons with VB.NET
Pay attention for the class
and see how it works in the code which comes with the article.