You can resize the dock by clicking between the applications and the section where the trash can is located. If you place the mouse between those you will get a set of arrows pointing in different directions. When that occurs click and move the mouse up or down to increase or decrease the size of the dock.
For future reference, you can reset your dock by deleting a file named com.apple.dock.plist and restarting. However, I think that would be overkill in this instance. 🙂