Fires will look better or worse depending on your graphical settings. Beyond that, some of the featured screenshots were taken with in-game bloom and motion blur post processing effects to capture the fire animation in a single frame.

As to fire sizes, they are dynamic and can change depending on what's being burnt - if you want bigger fires, you can always throw in more flamable entities. Fires on concrete or sand will not grow as much as fires on foliage or grass.