An echocardiogram is a good source to determine left ventricle dimensions, and EKG is not...many false positives and an athlete's heart size would provide an abnormal EKG output.
If you have an enlarged left ventricle as a result of vigorous aeorobic exerciser, you should have a pulse rate less than 60 beats per minute at rest, very good exercise tolerance, etc. If and when you reduce or stop the vigorous activity, an non-pathological enlarged left ventricle will/should return to normal dimensions.