The Shulhan Aruch (Orah Haim 210:1) brings an opinion which maintains that if one eats a "Beriya" – a whole product, in the original form in which it grew – then he recites a Beracha Aharona even if the product was smaller than the size of a "Ke’zayit." The classic example is a whole, small grape. A grape is a whole product in its original form, and so if one eats it without first slicing it, then, according to this opinion, he would recite a Beracha Aharona regardless of the grape’s size, even if it is smaller than a "Ke’zayit."
The Shulhan Aruch does not accept this opinion, but he nevertheless writes that it is preferable to avoid this situation by never eating a whole product that is smaller than a "Ke’zayit." The Gemara in Masechet Baba Kama teaches that one who wishes to be a "Hasid" (especially pious person) should be particular careful with regard to the laws of Berachot. In the spirit of this Talmudic teaching, the Shulhan Aruch advises avoiding the consumption of a "Beriya" that is smaller than a "Ke’zayit." Therefore, one who wishes to eat a grape should preferably either ensure to eat an entire "Ke’zayit," or slice the grape before eating it.
It must be emphasized that this applies only to products that are whole and in their original form, in which they grew. A whole cookie or cupcake, for example, would not qualify as a "Beriya," because it did not grow in its current form.
Other examples of "Beriya" include an almond, a cashew and a pumpkin seed.
The Rishonim debate the question of whether or not a whole olive qualifies as a "Beriya." Rabbenu Yona (Spain, 1200-1263) maintained that since an olive contains an inedible portion – the pit – it cannot, by definition, be considered a "Beriya," and even if one eats the entire olive with the pit, he is not considered to have eaten a "Beriya." By contrast, the Rosh (Rabbenu Asher Ben Yehiel, Germany-Spain, 1250-1327) maintained that eating a whole olive without the pit is considered eating a "Beriya," since this is how a whole olive is eaten. In between these two extremes is the view of the Rashba (Rav Shlomo Ben Aderet of Barcelona, 1235-1310), who held that if one swallowed the pit with the olive, then he is considered to have eaten a "Beriya," whereas if one did not swallow the pit, then he is not considered to have eaten in "Beriya." The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) sides with the view that olives are not subject to the laws of "Beriya."
Nevertheless, Rav Yisrael Bitan writes that it is best, if possible, to avoid this question by either ensuring not to eat less than a "Ke’zayit" of olives, or, when eating just one small olive, to first slice it.
Summary: If one eats a whole fruit or nut in its original form – such as a whole grape, or a whole cashew or almond – then according to some opinions, he must recite a Beracha Aharona even if he ate less than a "Ke’zayit." Although Halacha does not follow this opinion, it is preferable to avoid this situation by not eating this kind of product whole, without slicing it, unless one eats a "Ke’zayit." According to some opinions, this applies even to a whole olive, despite the fact that the pit is not eaten.