I've added my hand to this picture to give a guide to size. In fact this is a relatively small durian. They can grow up to 30cm in length.
This blog has been written for those who have never had the pleasure of meeting a durian. I say 'pleasure' because I do enjoy durian but apparently among Westerners I am in the minority. During the durian season many hotels in Southeast Asia have signs forbidding durian. The smell of it is so strong that once it gets into the air-conditioning system people may start checking out. I do find the smell pleasant but if you don't, it may be annoying. When one of my neighbours has brought one home, I know.
If you are in Southeast Asia and it's durian season perhaps you'd like to try it for yourself. The first suggestion is that you should not eat too much. Traditional beliefs say that durian has warming properties. If you eat too much at once it can heat up your body. Having said that I know that here in Malaysia it is possible to have a durian feast. These are often organised by the farms where they are grown. You go along, pay your money, and they will fill you up with a selection of different varieties of durian. I've known people who've done this and they didn't appear to suffer any ill effects. However for your first try, I suggest you have just a small amount.
When I lived in Bangkok a few years ago, during durian season there was usually a stall or two on the roadside around the corner from my home. They would have the durian already opened and the fruit packed in the right size for a serve. I remember paying about 30 baht, that's roughly $1. It seems to be a little more expensive here in Malaysia.
If you'd like to know more about durian, Wikipedia has a page that should answer all of your questions. It includes some delightful descriptions of the smell.