Transcatheter aortic valve implantation (TAVI) is a minimally invasive surgical technique to treat aortic heart valve diseases. According to current clinical guidelines, the implanted prosthetic valve replacing the native one is selected based on pre-operative size assessment of the aortic annulus through different imaging techniques. That very often leads to suboptimal device selection resulting in major complications, such as prosthetic valve leakage or interruption of the cardiac electrical signal. In this paper, we propose a new, intra-operative approach to determine the diameter of theaortic annulus exploiting intra-balloon pressure and volume data, acquired from a robotised valvuloplasty balloon catheter. An inflation device, capable of collecting real-time intra-balloon pressure and volume data, was designed and interfaced with a commercially available valvuloplasty balloon catheter. A sizing algorithm allowing to precisely estimate the annular diameter was integrated. The algorithm relies on a characterised analytical model of the balloon free inflation and an iterative method based on linear regression. In vitro tests were performed on idealised aortic phantoms. Experimental results show that pressure-volume data can be used to determine annular diameters bigger than the unstretched diameter of the balloon catheter. For these cases, the proposed approach exhibited good precision (maximum average error 0.93%) and good repeatability (maximum standard deviation ±0.11 mm).