I have really big or small hands. What can I do to make it easier to play?

Both small and large hands can make it hard to play unless you use proper technique.  Adjacent strings can be muted by improperly arched fingers with both large and small hands.  With small hands, reach can be a problem as well.

 There are three things you can do to fix this.

  1.  Arch your fingers more so you use only the fingertips. 
  2. Move your thumb's position on the back of the neck to allow more reach.
  3. Try a  new guitar with a more appropriately sized fretboard and neck.