Your new best friend will take a prominent space on your countertop. This means you'd want one that can fit in the space that's available. If your kitchen is expansive and you have ample counter space, then your coffee maker's size won't be of any concern. But, it would still be smart to take the exact measurements of the countertop space that you have before you purchase a unit.
Also, if you are leaning towards an appliance that's single-serve, you'd want to consider whether it'll accommodate your coffee mug or cup or not. You can check user reviews as they offer guidance on what can fit underneath an appliance's spout.