Would a cardiac mri of shown a enlarged heart and cardiomyopathies? what about heart failure?

Yes: Cardiac MRI would give very accurate info about chamber sizes and motion/function. "Heart failure" per se is a clinical diagnosis. BUT the cardiac MRI would be able to tell you how well the chamber is squeezing and whether its functioning normally or not.

MRI: Yes an MRI can be used to diagnose enlarged heart, cardiomyopathy and heart failure. Another test that is often used to make these diagnoses is the echocardiogram. MRI uses magnets to image the heart and echocardiogram uses ultrasound.