Potential for water damage is why I would never have wood flooring in a kitchen, bathroom, or laundry room. Ceramic tile will be harder and colder than the wood floors. They do make ceramic that looks like wood planking now. You need to resolve the transition from the existing to new flooring. Tile may require another substrate to prevent cracking.

We were thinking of taking all the wood out of kitchen, drawing a line from dishwasher to island, then a diagonal line from island to fridge - replacing wood with a ceramic tile. Is there a guideline for the size of the squares used in a small space?

Any guidelines about size or shape for ceramic or porcelain tiles for the kitchen? Should I be looking for something dark to complement the wood?

there aren't really any guidelines.
a rule of thumb that i've found which works well is that you want to be able to fit 3 or more tiles across the area without cutting, if you're doing a square lay.
so if you have 42" of walking space, you can use 12x12s without issue.
to make the area larger, you can lay them on the diagonal.

do keep in mind though, that tile floors are thicker than engineered woods, so you will need some kind of transition.

Would you say to try to bring out a color from the granite for the tile - that blends with the existing wood floor - or go for something completely different?