この小せェサルの島国にはいなかっただろう テメェを支配出来るボクサーは
I have nothing to add the explanation for the grammar of 「小せぇサル」 in Chocolate's answer.
But I think 「小せぇサル」 is kind of metaphor implying the person who leads the group.
Since 豊臣{とよとみ}秀吉{ひでよし}, daimyō, politician of the sengoku period, whose nickname is ”「小{こ}猿{ざる}」 : little monkey", there were the leader whose height is not tall in Japan, "140cm" according to this site.
Whereas basically athletes in Cuba basically compete only in amateur league and play in a national team. Therefore, the athletes should respect the leader, and the leader in Cuba was Fidel Castro, a tall person : "191cm" by google search.
It is not clear if the author wants to imply the average height of Japanese athletes is relatively small comparing to that of Cuban. But I guess probably the author tried to use some contrast in the page or the author had an images of the leaders to some extent.
So, I think the sentence from Cuban boxer is saying "Even little monkey like you lead the opponents in this island.". And probably in Cuba, which is also the country of an island, the situation is different.