Sizing for transcutaneous aortic valve replacement (TAVI) relies on non-invasive imaging. Incorrect sizing may result in adverse outcomes including paraprosthetic regurgitation, asymmetrical expansion which may impair prosthesis durability and, in the case of severe sizing errors, device embolisation or aortic root rupture. This review addresses the optimal approach for sizing. It is based on currently available data including the anatomical implications of using different imaging modalities, the steps of how to reproducibly measure the correct annulus diameter on MSCT, and current evidence for different sizing strategies.