In the field of ultrasonic testing there are two key questions: Which defects can be found and-in the case indications are found-do they restrict the use of the part? Regarding both questions, the prerequisite is a method for defect sizing. Over the last decades sizing methods were established like DGS (Distance Gain Size) or DAC (Distance Amplitude Correction) for defects smaller than the beam profile. Those methods utilize the echo amplitude and provide results which are proportional to the defect area. However, those approximations are only accurate for defects larger than one wavelength even that experience shows it can be applied for slightly smaller defects. With the progress of material technology and ultrasonic inspection the need to detect and size smaller defects is growing. Therefore, both for flat bottom holes and disc shaped reflectors the usability for small defects needs to be checked. In this publication, it is investigated how to correctly size small defects below one wavelength. Utilizing a grid-based simulation method the echo signals of cylinder and disc shaped reflectors of various sizes are calculated. By properly choosing the simulation method and grid it is ensured that all physical wave modes are included in the simulation and that the discretization error is negligible. A good correspondence between the simulation and classical defect sizing for defects larger than one wavelength is found. In the region between one quarter of a wavelength and one wavelength resonance effects are found, which result in classical defect sizing methods giving conservative results. In the region below one quarter of a wavelength classical DGS and DAC sizing leads to undersizing. This is discussed in detail and a formula for defect sizing is derived, which is applicable to small as well as large defects.