Does 'Cast Duration' on a spell within the creation kit expand it's effects, such as a fire damage magic effect in a spell with a duration of 1 second is played further if the cast duration is longer than the duration of this effect, increasing the damage? Or is the animation for the spell just played longer or something?

I did some google searching and some searches here and found no definitive answer, the creation kit website is also vague on this aspect, I would test it myself but I am swamped.

Cast Duration is how long your character will wave their arms around before the spell is actually cast. The spell effect is not in place until it is cast.

This is sort of an ambiguous question, since no such thing as "Cast Duration" exists in the CK. If you mean "Casting Time" (which is found in the Magic Effect window), the CCNA is correct, it affects the amount of time you must hold down the casting button before the spell is fully charged and ready. If you mean the "Duration" of the Effect Item (which is the single line entry for each effect inside the Spell window), that affects how long the spell is active. A spell will cause it's "Magnitude" in damage every second of "Duration" it is active, so increased duration usually means increased damage for most spells.