Please suggest how to change related post image size at the bottom of post
To change the size of the images used in portfolio/blog, please navigate to theme options -> blog, portfolio & shop -> featured image.
Remember to use thumbnail regeneration after you make your changes. Otherwise, the images will not change their size.
But it will also change the featured image size and so many option doen there with radio button in the option given by you.pls suggest
It is the only place where image size can be changed.
If you want to change only related posts images, you will have to modify the files, but we do not provide help with such customization.