I used over size pulls on my cabinets. For my drawers, I made a "rule of half". I used pulls that were approximately half as long as the size of drawer. So pretty much the same as ctycdm has. In the picture below the drawers on the left are 24 inches wide with 13 inch pulls. The 36 inch drawers in the middle have 18 inch pulls and the drawers on the right are 18 inches with 10 inch pulls. I also have some 13.5 inch drawers that have 6 inch pulls. Since I like the look of longer pulls, I used 10 inch pulls on the cabinet doors.
If you google using the words gardenweb, pull size and drawers you will get lots of posts and pictures. Many GWers have only one smaller pull while others have 2 smaller pulls on wide drawers. It's completely a matter personal preference and sizes available in the hardware you like.
Q