A mi medida literally means "to my measure". It's a wonderful idiom used to say something is a perfect fit, as if it were custom-made.
In the song, the singer describes his partner as being "hecha justo a mi medida" (made exactly to my measure). He's using this phrase to express the feeling of finding a soulmate, someone who fits perfectly into his life. It’s a much more poetic and heartfelt way of saying "you're perfect for me."
This song is a huge, heartfelt declaration of love and gratitude. The singer, Julión Álvarez, feels like he's hit the jackpot with his partner. He sees her as a perfect match, saying she's "como si estuvieras hecha justo a mi medida" (as if you were made just for me). The best part for him is that he didn't even have to look for this love; it just appeared in his life. He calls it a "regalo de Dios" (a gift from God), something so special because it was an unexpected surprise.
In return for this amazing gift, he promises to dedicate his life to her. He sees it as his job to look after her, making it his "única misión será hacerte muy feliz" (only mission will be to make you very happy). He promises to be there through good times and bad, and all he wants as a reward is simple: "mirar tu linda carita sonreír" (to see your pretty little face smile). It's a classic, romantic Norteño song about finding your perfect person and promising them the world.