Mildly enlarged liver 18cm echogenicity and contour normal all blood work normal. could this just be my normal liver size? how enlarged is 18 cm?

Maybe: Most normal values depend on the measurement being obtained at the "mid clavicular line" , with upper limits of normal at 16cm. The liver comes in many shapes and sizes however, with many normal variants.

Maybe.: How large are you? If you are small or average height and weight then your liver is mildly enlarged. If you are a taller and heavier person, then your liver size could be your normal. With the normal bloodwork and ultrasound, i would not expect that you would need a biopsy in the absence of other symptomatology.