The aperture size is a property of the lens only and does not depend on the crop factor. It does depend, however, on the actual focal length of the lens (not the "equivalent" focal length).

So you need to obtain the actual focal length by dividing by the crop factor.

$$f_\text{actual} = \frac{f_\text{equivalent}}{\text{crop factor}}$$

You can then calculate the size (diameter) of the aperture (strictly speaking the size of the entrance pupil, which is the image of the aperture) by dividing the focal length by the f-number N (which does not need to be divided by the crop factor).

$$\begin{align} \text{Aperture diameter} &= \frac{f_\text{actual}}{N} \\ &= \frac{f_\text{equivalent}}{N \times \text{crop factor}} \end{align}$$

So in your case the calculation is:

$$\begin{align} \text{Aperture diameter} &= \frac{50\,[\text{mm}]}{2.8 \times 1.5} \\ &= 11.9\,[\text{mm}] \end{align}$$