Sizing of indications and the determination of the inspection sensitivity are core tasks in ultrasonic testing. They ensure that critical inhomogenities can be detected and assessed. For the Synthetic Aperture Focussing Technique (SAFT) the sizing of indications is currently performed by evaluating the spatial extent in the reconstruction result, i.e. counting the number of voxels belonging to an indication. This limits sizing to the resolution of SAFT, which is in the order of one wavelength. For smaller defects sizing is not possible. Moreover, no information about the sensitivity of the inspection method is provided. Compared to classical UT inspection SAFT significantly improves the signal-to-noise ratio (SNR), regarding stochastic noise and grain noise. However, this improvement of the sensitivity cannot be utilized adequately without a method for determination of the sensitivity and for sizing small indications. Similar to classic UT there is a second option to gain information about indications: The amplitude sum, which incorporates the echo amplitude, the angle dependent scattering characteristics, the probe parameters and the specimen geometry. In this paper we show that the amplitude sum is suitable for sizing of small indications and for the determination of sensitivity. The relation between the amplitude sum and the size of indications is explained, and it is shown, that the location of indications and the shape of the object to inspect must be considered. Based on these results a method for sizing of small indications using SAFT is developed which translates the amplitude sum of each voxel in an equivalent reflector size, similar to the Distance Gain Size method (DGS) in classical UT. This sizing method completes SAFT, growing it from an imaging tool to a full-fledged quantitative measurement technique.