Came across this interesting fix. Not sure if there is a reason not to use it, but it helped us achieve equal heights in the testimonial slider. Basically just changing the display property of the testimonial row and the testimonial item. Hope it helps

#top .avia-slider-testimonials.avia-testimonial-wrapper .avia-testimonial-row {
display: table;
}

#top .avia-slider-testimonials.avia-testimonial-wrapper .avia-testimonial {
display: table-cell;
}