A good rule of thumb is 1/3 of the door/ drawer length.

I like a larger pull look so I used 5" for my lowers which range from 15"-24". Very thing else, I used 8".

Go pick up a couple of different size pulls (handles) at a home improvement store. Take them home and see what you think looks and would feel right on the cabinets. This is just for size, not for what you would really put on there. If the cabinets are not in yet, go to the cabinet displays with your samples and check it out.

Whatever you choose, try to get a sample in your hand before you order them all or have them installed. I fell in love with a certain knob. Unfortunately, the matching pull had very little space for your fingers. I ended up choosing a different pull by the same manufacturer in a slightly different style but the same finish.

I used knobs on almost all the cabinet doors (the hardware I picked was not cheap, and using pulls everywhere would have been very expensive, there are a lot of cabinets). On the normal sized drawers, I used 4" pulls (Center to center hole measurement). I have two stacks of wide drawers, and I used two 6 3/4 pulls on each of those drawers. I also used the larger pull on the trash pullout and a tall pantry door. I have dark raised panel furniture looking cabinets so I think it looks nice, and I like the way they feel.

If you have shaker or a more modern type cabinet, using the same size pull size on everything might be a better choice so you get a sleek consistent look.

I did not put pulls on the dummy drawer below the sink. I did not want anyone to mistakenly try to open it, and felt that it would be in my way when working at the sink.