Hopefully enjoying good health.
How to successfully implement FlexSlider or the Nivo Slider,
It is just a matter of choice.
I was thinking maybe I needed to adjust the size of my images
No, you don’t need to adjust the size of image for different sliders. Only requirement is at-least ( 940 x 450 ) image dimension for slider post. Bot Nivo Slider and Flexslider will work best.