The section that you're using is a fixed height that accommodates all of the content without scaling vertically (i.e. not getting shorter and taller based on the content of each testimonial). It also vertically centers the testimonial content. What this means is that when you have testimonials that vary greatly in length the long ones will fit the space well and the short ones will have enough padding equal to the difference in length of the testimonials.
That paragraph wasn't exactly a solution, but there isn't an easy solution to preserve a good user experience (you don't want the section getting bigger and smaller as the arrows will keep moving up and down and out of reach). Typically the best thing to do is to have the testimonials in a section like this the same length (or similar at least) and if you want to have the full long testimonials then store them all on another page (which I see you already have). Treat these as tasters rather than the whole quote.
Hope that helps!
