Size is a perreo flex track, plain and simple. It opens with a big boast, "Yo soy el pasado, el presente y el futuro" (I am the past, the present and the future), and then spends most of its time on one woman.
The hook is the catchiest part, mostly because it mixes Spanish and English words that rhyme: "tus Nike combinan con tus eyes" (your Nikes match your eyes), "el cuello lleno de ice" (neck full of ice, meaning diamond jewellery) and "cintura chiquita, mami, de perfecto size" (tiny waist, perfect size). Then comes "es de familia pelolais", and pelolais is Chilean slang for a posh, rich, usually blonde girl, which is funny next to "le gustan bandío'" (she likes bandits). Good girl, bad boys, you get the idea.
Be warned: the verses get very explicit fast, with lines like "te lo meto toda la noche" (I do it to you all night) and gun talk such as "la .40 está chipeá'" (the .40 is chipped, slang for a gun modified to fire automatically). Notice the dropped letters everywhere: chipeá', guillao', desvelao' are chipeada, guillado, desvelado, the way the words actually get said in street Spanish. The ex who "está pelao' y sigue pelao'" is broke and still broke, a petty little jab that comes back every chorus.