There are two ways to resize the sprites, if you have them but want to make them a different size.
1. Use a program like Paint or Photoshop on each individual frame.
2. Put HIGHRES = 1 in your S_SKIN Lump.
The first one is harder, but it usually produces much better results than the second one. Also, the second one can only be used if you want to make the frames exactly HALF their original dimensions.