thumbnails to slider bullets do-able

I am using this plugin, though it is not updated for 4.0 it is still workinng and just what I need. But when I try to re-size the thumbnails to custom they get huge. I was wondering if there was a way just to have it show bullets for each image.

Not sure if the creator is still working with this but would love to be able to use it .

Thanks

https://ww.wp.xz.cn/plugins/post-gallery-slider/