All the Post slides on that site are 400px tall, at least from my machine, Win10 on Chrome. I think the problem is with the excerpts for each. It is counted by the number of words, not letters, so it’s hard to make them all uniform, unless you want to play with the wording. You could try this CSS snippet I have though to limit the height of the excerpt and hide all the extra words. Feel free to adjust the max-height value to suit your needs.
.fl-post-slider-feed-content p {
max-height: 105px;
overflow: hidden;
}