It's such a personal choice and anything goes as long as you like it. A lot of people, including myself, have knobs on doors and pulls on drawers.

5" is fine, looks good with most widths of drawers and cabinets. I always felt 3.5" were awkward, too small for grasping comfortably but might be necessary on very narrow drawers. If you can use a wider pull on larger drawers (I didn't because the larger size was so much more expensive), I would do that instead of 2 smaller -- buy one or two extra if it is an odd width in case you need to replace? Placement really is to individual taste. I put all of mine on the top frame (of the shaker drawers) and vertical on the side of doors. Not all drawer construction can take a screw evenly through the top frame, though, look at the backside of your drawers to see. I also like pulls placed just a bit under the top frame, and the same on all drawers. I don't care for having the pulls centered in the center panel so much, especially when drawers are different sizes.

I don't think rules are hard & fast either way. But I agree, proportionately your doors would look better with pulls. Regarding knobs, in my remodel I stayed away from using knobs where my clothing could get caught on them. In my old kitchen my belt loops or apron were often snagged on the top base drawer knobs. I'd consider 3katz4me's suggestion.

"I think the premise is to have pulls be *no more than* 1/3 the drawer length." I think designers invent "rules" like this just to have rules. I have never paid much attention to such nonsense. There is a limit, of course, to how wide pulls can be. You don't want them to interfere with drawer operation. Other than that, anything that looks good to you is just fine. As for variations in length, don't worry about it. There is no rule that all pulls have to be the same size, although the same general design is good. One thing about pantry pulls. You are probably going to be opening pantry doors with arms full of groceries, so a pull you can operate with an elbow, wrist, knee, or one pinky finger is a good choice.