Displaying latest Posts – fixed height – Excerpt vs. Content?

The simple way to do this is to define the height/width in CSS of a container, and then apply a trim on the excerpt.

To change default excerpt length add this to you functions.php

function new_excerpt_length($length) { 
    return 100; // change this to how many any characters you want
}

add_filter('excerpt_length', 'new_excerpt_length');

Manually controlling the excerpt
How to control manual excerpt length?

Outside th loop
Using wp_trim_excerpt to get the_excerpt() outside the loop

The same filters can be applied to the content, but the excerpt suits this better.