I don't think there are hard/fast rules for sizing hardware.

My mantra would be: avoid extremes. Know that certain proportions tend to look better to most people's eyes.

Consider first the performance and function. If you have a large door or drawer and a tiny pull or knob, it will be difficult to open, for example.

Next, consider appearance.

Why not cut out some cardboard or something in the different length options, tape them up or hold them up to a drawer (or a mock-up of a drawer) and see which ones look best to your eye. Since there are no major rules, then go with what looks best to you.

I've seen cabinets with super long/tall pulls. It's a look. I, personally, don't love that look, but the next person might. Like this:
This style isn't hurting anyone, but it might not appeal to many people.

The other thing to consider is that you're putting dark pulls on espresso cabinetry. Whatever size you choose, it is not going to stand out as much if you get the proportions wonky. Best wishes for success with your kitchen.

You might want to pick some knobs of the same line for the upper cabinets and the 10" drawers. Then you might use the 125mm (that's roughly 5") pulls for everything else. You will even get away with one pull per 31" drawer. If you don't like that idea, you can get away with putting the 5" pull in all places, even the 10" drawers.
It also depends on the size of your kitchen and the massiveness of your cabinets.

The next issue is, which AMN already touched on: You picked black cabinets and dark hardware. What is the style of your kitchen cabinets? Don't you want to show up your hardware a bit and maybe go with brushed nickel or stainless? What color of appliances do you have?

The longer your pulls are, I mean more than 12" or so, the more contemporary the look is.

I think a 31" drawer with a 5" handle looks disquieting. I would rather do without. I personally think a wider drawer needs a wider handle because it seems more in balance.

We're having some work done in the kitchen. I decided to order knobs for the upper cabinets, a small handle for a very small drawer, medium handles for medium drawers, etc. The handles are ones that are meant to be installed horizontally so I couldn't use them for the upper cabinets but I also wondered if all those handles would be too 'busy' in my small kitchen. I bought one of each and tried them before ordering the number I need (the special order was for some reason, less expensive for the same thing). I agree with carolannr - I think you need wider with wider, etc.

Like I said before, it entirely depends how spacy the kitchen is. The 5" would be the minimum for a 31" drawer. You can also use two.

Jan, what a timely question. I'm facing the exact same question for all our cabinets (kitchen and 2 bathrooms). I have four very wide drawers in the kitchen, around 35" (89 cm) or even wider. Similar to mmers' ideas, I'm considering no hardware for the upper cabinets, and only have pulls for the drawers. :)

Thank you all for the input! Think I'll go bigger than 5" on the big drawers. I like the idea of knobs on the little drawers because the small pulls are kind of a pain - get your finger stuck in them. My appliances are stainless but I'm trying not to emphasize that. All the fixtures, lighting, and doorknobs are oil rubbed bronze. The cabinets are dark brown ( not black) so I was thinking black handles so they would show but not stick out. I'll put a pic in of the kitchen in case anyone else has more thoughts. Thanks all!!

Since you have all the appliances in stainless, you can well go stainless or bushed chrome, or brushed nickel for the knobs/pulls. Even if all the fixtures, lighting, door knobs are oil rubbed bronze.

Jan, what's the name of the puppy ? :)

Very pretty kitchen. I think of the hardware as the jewelry of your kitchen, so I would go with brushed nickel to accent your appliances. Id suggest you bring home a few samples of several colours before you order and check them out for a few days. The hardware doesn't have to match your lighting fixtures, it's ok to mix it up a bit.

I'm curious to know how the cabinets turned out?

Do the knobs horizontally for a natural pulling motion.

I just did mine and I went with about 1/3 the size of the drawer. So maybe the 3" for 10" drawer, etc.
I used a similar Amerock style. I also have different size drawers and I so bought 3 lengths I believe. I used amerockforless.com.
I like how they look.

Karen, thanks for answering the question. I'm about to purchase one long drawer pull so that I can open long base cabinet drawers with just one hand. I too thought there must be a rule of thumb for the ratio of width of pull to width of drawer.

So timely, I just bought 2 different size pull bars for our new kitchen cabinets. I was questioning using the two smaller sized pulls horizontally on the drawers. I think I will take ajanthony's advice and use only one larger pull for the drawers so they can be opened with one hand as I often have only one hand to use when I'm opening a drawer. Thanks!

Shannon Lewandowski, do you mind showing me a photo of your different handle sizes? I'm in the middle of the same predicament right now... choosing handle sizes for different sized drawers and cupboards and not sure how it would look with varying sizes of handles. Thank you!

I did the painters tape too and it really helped.

Sorry for the delay @mrsjoannelee! Here's a picture from a few weeks ago with some of the different lengths.

Any suggestions on where I have 3 different length drawers and picked out cup pulls and then I have varying door sizes and picked out knobs? Will the knobs look odd on the larger doors?

@Shannon Lewandowski where did u get those pulls from?

I'm doing the same right now. Trying to pick out hardware. I am very sensitive to balancing, I am not working with a designer. I think with the help of other houzzer's, I will go with oil rubbed bronzed pulls. I'm thinking 8" or 10"? Opinions? I am adding a crema marfil backsplash.

The second photo is cabinets similar to mine, and I like the look.

No, you pick out what you like best. Take a sample home, or take a door/drawer with you to the store.