DS is 13 months and a finger-food "expert" but I still cut grilled cheese into pretty small pieces. I also use a pizza cutter to make it easier. He had one for lunch today!
At first I cut them up into small bite-sized pieces (like a puff or piece of fruit). Now as she gets older the pieces are a little bigger. I could start to tell that she could handle bigger pieces. Just follow your LOs lead. Try a certain size, see how they do, and adjust accordingly.
This seems crazy to me! I cut it in half and he takes bites, and has for a couple of months now. I don't cut the crust off either. (Should this be a FFFC?)
Re: serving a grilled cheese sandwich as finger food?
This. That way DS can hold it and break off pieces.
This. He loves it!