The pulls should be approx. 1/3 the width of the drawers. So, either 8" or 9" pulls for what you have.

Thank you! Should I pick same size for both, and if yes, which is better- a little bigger or a little smaller?

Typically pulls/knobs manufacturers won't make both an 8" and 9" size so you won't likely be able to have both in the same style. Look at the pulls you like and see if they come in either of those two sizes. I usually feel that pull sizes should vary in the kitchen with the size of the drawers, but that is more if the drawers are quite different in widths. Your drawers are not so the pulls can be the same size.

A good trick is to make aluminum foil pulls in the sizes you are looking at, and hold them up against the drawers. That will help you see which size works best.

24" and 27" drawers should have same sized pulls. I agree with about 1/3 of the width. For us we have paneled drawers so went about 1/3 of the inside part of the panel and looks great.