The current standard is 3" x 5" unlined index cards bisected widthwise. However, variants exist for pragmatic reasons: lined cards provide an easily identifiable back, full cards provide more space, A7 cards can be made extremely easily out of A4 paper, etc. The cards may not even need to be white, or even totally blank. If you want to go really crazy, maybe not even cards in the strict sense of the word.
It is normal convention (Or to put it another way, the point of the game) to draw upon these blank white cards any picture, title, effect or other thing they wish. It is not unheard of to modify the cards by taping things to them or mutilating them.
A blank white card can usually be considered a far better draw than an already drawn-on card, as it can be manipulated to solve any problem the player faces, be it a particular card, a group of cards, or an opponent's refusal to shut up.
There are three basic "series" of cards in any given deck, classified depending on the situation they were created in.
First series: Fresh IdeasEdit
These are cards created for not really any reason other than that the creator has a cool idea for a card (which we all get from time to time). These tend to be of fairly high quality, and generally occupy the top 3 levels of Professor Ickbweck's Sliding Scale of Artistic/Notational Tendencies, as the effect often takes a back seat to the art.
Second series: Cards of the NowEdit
These cards are the ones that occupy the rest of the layers of Professor Ickbweck's Sliding Scale of Artistic/Notational Tendencies. They're created when somebody has a problem they need to solve. Often lack titles for some reason, and are usually of slightly lower quality than First Series cards. A great deal of these are Card Manipulation Cards, because quite often players will want to discard boring cards from their hand or remove irritating cards from play. These are created to get the player out of whatever trouble they're in at the moment.
Third series: BlankfillerEdit
These cards tend to be the lowest quality of all, although not always (Sometimes they can be great and popular) . These are created when someone:
A. Has a lot of blanks in their hand.
B. Lacks inspiration.
These can fit anywhere on the sliding scale. Sometimes someone just slaps a point value on an otherwise blank card, and sometimes they will just start doodling hoping that inspiration will strike.