When measuring ascending aorta size (horiz)from sinotobular junction to innominate artery is cardiac mri/mra for morphology more accurate test than cta?

Depends. Done properly, ideally with ECG gating, an aortic MRA is equivalent to an aortic CTA. The opinion from some vascular docs is an overestimation on MRA. This was probably from movement artifact and older scanners. The benefit of MRA is the lack of iodinated contrast and lack of radiation. It is in routine use at many large medical facilities to periodically track aortic enlargement- kids/adults.