It's important to choose a banana of normal size.. not too big, not too small. If you have a dick you would like to deepthroat in mind, it should be about that size.
Some dicks are longer, some dicks are wider, your throat should normally be able to take any dick size, but if you're a beginner you should start with something small. It is best in fact to start with something a bit more thin rather than wide. The length is not really that important, since you won't be shoving it all the way the first times..