I just got my first piece cut in. It covers the hairline area to give me a straight hairline. The piece is located only in the area in front of the ears. My stylist said it would be easier and more convenient if my next piece reaches back to the top of the head behind the ears. Is that true? Would it look better and be more/less comfortable?

It is true if your hair back there is thin. I think it is good to use as much of your own hair as possible, however, if the hair is thin, it is harder to blend the partial frontal piece into it.
you don't want thick hair in the front graduating into your own thinner hair, that would look "off".
so in order to answer your question on what size base is best for you, we would have to see photos, as everyones balding pattern, thinning areas are different.
Your stylist would have nothing to gain by telling you to get a bigger piece.....so I have to believe that perhaps they thought it might look more blended or just a better overall look with your own hair to have a larger piece.
DebbieS.