Are there any big cells, and if so how are they modified to be so big yet able to function?

Yes, there are some big cells present in the body such as white blood cells and microvilli in the small intestine.

Explanation:

White blood cells have big size which enables them to produce more RNA and proteins which are needed by the cell. Microvilli in small intestine also have a big size which enables it to absorb more nutrients from the food present in small intestine and add these nutrients to the blood which is used by the body.