Yearly ECHO is a good way to follow the aneurysm size. Most radiologists are very good about the measurements. Even the cardiologist rarely look at the actual images, but rely on the radiologist read/report. Your PCP can order the test and read the report. The 5 cm size threshold has been debated by the surgeons. If the size creeps up slowly, then when do you need the surgeon?
They seem to wait until your valve goes to enter the chest. But since you already have an artificial valve....I would definitely follow the size.
FYI: I just had mine repaired, along with a bovine valve. It took nearly 10 years to go from 4.7 to 5. What an experience!