What exam are you referring to? If you are refering to the CPC exam, it doesn't cover billing per se.
The CPC exam has 150 questions and is a comprehensive exam. There is no one area the can be called the "biggest part" of the exam. There are questions on the 10000 series, 20000 series, 30000 series, 40000 series, 50000 series, 60000 series, radiology, path and lab, Medicine, E/M, anesthesiology, medical terminology, anatomy, ICD, and practice management.
Generally there are - give or take- about 10 or so questions from each of these topics. So it is best to study all of these areas. A good medical coding course will cover these topics. Then you can take some practice exams to identify areas where you need to devote more study time.
BTW, after a medical coding course where you learn about the CPT and ICD codes, the Blitz videos are very helpful for preparing your manuals so you can find the appropriate codes readily. For more information, c
lick on this link: Blitz Videos.