What causes enlarged liver (20.5cm) with normal LFTs? Only abnormal labs in ER were low creatinine (0.5) and low BUN (5). No masses on CT. 24yo, BMI 25, no etoh. What should I ask my dr next week?

Enlarged liver. As a start, someone needs to reevaluate that test and determine what portion of the liver was measured. 20 cm at the midclavicular line is large, but it is not in other planes. In the US, the most likely reason would be fatty liver, which is supported by the BMI of 25, which the NIH would say is overweight, but you need to be evaluated by a hepatologist to be sure nothing else is missed.
Can be normal. A few percent of folks have an extra liver love that gives a falsely-elevated span. I would not worry in any case.
See below. How do you know that your liver is 20.5 cm? if that is so, your liver is enlarged and that is the only question you need to ask. Your lab test results are normal, do not be concerned about low creatinine and BUN. What you and your doctor need to ascertain is why is the liver enlarged, if it is really 20.5 cm.
Is it real? Tell us why you say your liver is enlarged? This is a difficult diagnosis to make unless the liver is more than 50% bigger than normal. The causes of a large liver, besides hepatitis include infiltrative diseases like Amyloidosis, Fatty liver or a lipid storage disease(some of these are hereditary(so check your family history of Live). A liver biopsy is the best way to find out what is the cause.