The gemara makes it clear that without kezayis there wouldn't be any bracha made. Tosafos qualifies the gemara to be referring to the bracha achrona, but when it comes to bracha rishona, Tosafos writes that we make it even on less than a shiur. The rationale of Tosafos is that the source for bracha rishona is אסור ליהנות בעוה"ז בלא ברכה, therefore, one must make a bracha rishona even on a tiny amount.
This approach seems difficult because the gemara on 35a was discussing both bracha rishona AND bracha achrona, yet according to Tosafos we reject the kal v'chomer and rely on the sevara of אסור ליהנות בעוה"ז בלא ברכה for both bracha rishona and bracha achrona. If this sevara is mechayev both bracha rishona and bracha achrona, why should bracha achrona be bound to a shiur kezayis and not bracha achrona? It seems that even if we use the sevara as the source for bracha achrona, chazal still modeled the bracha achrona after the din d'oraysa which is that we require an achila of kezayis. Although it is not exactly modeled after the bracha achrona d'oraysa because they instituted that bracha should be said even on a kezayis, even without se'viah. That is because se'viah is very subjective so they couldn't be magdir bracha achrona practically based on seviah. Instead they said that whenever one eats a kezayis they must make bracha achrona and if they happened to be satisfied from that eating, the bracha achrona they were making would actually be d'oraysa. But if they didn't allow you to make bracha achrona unless you reached שביעה, it would be impossible for people to figure out whether they met this requirement.
Bracha Rishona is made even on the smallest amount, provided that it is enough to provide a person pleasure. If one were to eat a crumb and could barely taste anything, they wouldn't make a bracha rishona, but if they at enough to receive some enjoyment, they would make a bracha. Tosafos doesn't say what bracha rishona is made, which implies that when one eats less than a shiur they make the standard bracha rishona appropriate for that food. However, Rabbeinu Yona writes that since what compels the bracha rishona on less than a shiur is not the nature of the food, rather avoiding benefiting without making a bracha, whenever one eats less than a shiur the bracha is shehakol. The Beis Yosef is medayek from the Rosh that on less than a shiur one should make the appropriate bracha for that food because when speaking about the bracha achrona, the rosh assumes that if one can't make the bracha appropriate for that food, there would be no other bracha to make. Similarly with bracha rishona, if one wouldn't make the appropriate bracha, there would be no other bracha to make. There is one opinion cited in Beis Yosef (kol bo) that even bracha rishona requires a shiur.
Regarding bracha achrona on less than a shiur, Tosafos cites the R"I who suggests making borei nefashos. Tosafos disagrees since the bracha on fruits is modeled after the ברכה אחת מעין שלש, therefore it should also require a shiur. The rationale of the R"I would be that only brachos which contain the term "achila" require a shiur of kezayis because that is the definition of achila. Borei Nefashos which doesn't use the term achila wouldn't require a kezayis.
Another issue is the shiur for bracha achrona on drinks. Tosafos assumes that the shiur is מלא לוגמיו and therefore encourages one to always drink at least a מלא לוגמיו from a כוס של ברכה so that they can make a bracha achrona. R. Akiva Eiger in the Gilyon HaShas references Tosafos 49b who also cites the R"I and says that one should be machmir to make a bracha achrona on drinking even less than a מלא לוגמיו (presumably they me an a kezayis would be the shiur for drinking as well). Aside from the Gilyon HaShas pointing out that being machmir in brachos is against the standard rule (see Tosafos 12a citing the R"I who says there also we go l'chumra to make brachos out of safeik), R. Akiva Eiger is also pointing out that the R"I in our Tosafos seems to make borei nefashos on less than a מלא לוגמיו, whereas on 49b sounds like you would make the standard bracha.
The Tzlach finds this entire shiur of מלא לוגמיו to be strange. Tosafos in Yoma 79a offers possibilities as to what the shiur is for drinking. Either kezayis, or k'beitzah, or Revi'is, but not ma'lei lugmav. It seems to me that Tosafos wasn't sure if kezayis is sufficient but he was sure that the shiur couldn't exceed malei lugmav. On Y.K. the shiur of eating is a large date which is larger than the shiur for bracha, so the R"I understands that for drinking the shiur for bracha may be the same as Y.K. but it couldn't be larger than Y.K. which is malei lugmav. This is clear from Tosafos 49b citing the R"I to make a bracha on less than malei lugmav since we see that the shiurim for bracha are smaller than y.k. Therefore, even if in our Tosafos there is a possibility that the shiur should be like Y.K., it would be impossible for the shiur for drinking to be larger than Y.K. since the shiur for eating is smaller.