The Rema (319:3), based on the Terumat Hadeshen (R. Yisrael Isserlin, 1390-1460, Austria), writes that two types of fish are considered distinct entities and the laws of Borer apply to selecting one type of fish from the assortment. Even if the pieces are large and easily distinguishable, one may only select the desired type, with his hands for immediate use.
The Rema continues and rules that if there is only one type of fish in the mixture, with an assortment of sizes, the laws of Borer do not apply and one may pick out the undesired fish. However, the Mishna Berura cites the Taz (Rabbi David Segal, Poland, 1586-1667) who is strict even in a mixture of one species. That is how the Chesed L’Alaphim rules, as well.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Beshalach par. 2, writes that even though the majority of Poskim are lenient (most likely including the Shulhan Aruch), it is fitting to be stringent, yet one who is lenient has upon whom to rely. Hacham Ovadia, in Hazon Ovadia 4, p. 188) writes that the Taz is a lone opinion, and the Mateh Yehuda argues that this Taz is a novelty. Therefore, the Halacha is lenient, and the laws of Borer to don’t apply.
However, in cases where the same species has a variety of flavors or tastes, such as apples or dates, the Ben Ish Hai rules that the restrictions of Borer apply.
The laws of Borer apply to a mixture of different types of fish, but there are no restrictions on selecting from a mixture of different sizes of the same item.