@ewei6911...Hello....
From reading here, you'll find out ct scan readings are most accurate, others not as much, BUT, it also depends on who reads it to evaluate size! Some use "inner diameter", others use "outer". Ultrasound/echo are also used, with possibly other resulting measurements. It's important to do the same scan procedure at the same facility for at least a few times to get a consistent and proper baseline. In my own case, I had a recent ct scan that was read by a radiologist at 4.9cm, and my cardiologist remeasured at 4.5cm. I'm having a "gated ct" to get a clearer scan picture next month for comparison, as recommened by Cleveland Clinic. A chest xray that you mentioned probably is the least accurate for measuring.