The specific size is less important than the design and proportions in context. "Rules" are generally conventions and some of the most effective designs break the rules.

Can you print a photo of your drawer pull at whatever size you are considering and tape it onto the remaining cabinets? If it is a super simple style like a bar, you could just draw a line on paper the length and thickness you need and tape that up. If you love the proportions you have started with, sharply changing to a shorter proportion that your sister thinks is appropriate may not help the room work as a whole, nor please your more dramatic tastes for your kitchen.

Hi I know it’s been a while but what did you end up doing? I’m considering long pulls for my somewhat modern white flat panel kitchen. I was thinking 12” (or longer) for the drawers and 8” for the doors. It’s a tough choice when buying on line and don’t want to deal with returns. My cabinets are being installed in 3 days! I need to decide and order ASAP and am having trouble deciding. Thanks!