Generally, a side deck is used to deal with, and counter decks that your specific deck has trouble against. For example, macro, or just plain things that banish, are a huge pain for fire kings, thus, we normally side imperial iron wall. Only when you've sided for those problem cards, do you side cards to compete and possibly shut down the rest of the meta. Examples for those were given by Gluka, but I'll name a couple he missed.
Droll&lock bird( For E-drag, or any deck that searches allot)
Mind crush: I am suprised this was not mentioned, as it is a veeery useful side this particular format, as it works against prophecy(When they reveal for priestess, or what they search) Mermails (abyssmegalo or again, searches) Fire fist(Again, searches) Evilswarm(Searches, like that pesky infestation quickplay that is like lance, or what ever Ophion searches out.) In short, if you can side this card, it will and should be useful if you know when to side it.
Debunk(Darkworlds, this is pretty self explanatory, other similar cards include the above cited Mind and soul drain.)
Maxx "c": (E-drags or anything that specials allot or swarms such as Dark worlds and others.)
Thunder king rai-oh: (This card was put to 2 because it would be so good this format, side it in, stops searches and special summons, and will act as a good card to sit upon and prepare things and whatnot.)
Effect veiler: can also be a great side card.
EEV/DDV: If you can use them, may be a good card, though EEV will be more useful, particularly against prophecy.
Prohibition: This card is very good and can normally effectively shut down a deck, call the search card for prophecy, call alius for heroes, the list goes on, but basically you call the card that hurts.
Now I guess to finish with your question, you should always have 15 cards in your side deck to be able to compete whatever eventuality befalls you, as for the test, they don't rate you on if you have a side or not, but whether or not you side effectively. So Side well against what you are facing and you should do fine.
"get your game on!!!!!
you made them cry... nothing can save you now.
" Quoth the raven, `Nevermore.'
I am infinite, for should you destroy me, I would come back stronger.