Answer:
I am not a medical professional, so if you have any concerns I suggest you seek a medical professional. The information I am going to provide is just based on my knowledge and should not be taken very seriously, or determine anything for you.
Though to answer your question, the normal size of a healthy, original tonsil is 2.5 cm in length, 2.0 cm in width, and 1.2 cm in thickness according to some sources I have found. Since you are 18, and tonsils do start to shrink in adulthood, compared to that quarter I would say that the oval shape you drew looks like a healthy size for a tonsil. However again, please do not take my word for it, as I do not want to spread misinformation, though this is just what I believe.