My kitten is so small, and yet shes so old!?

My 5 month old kitten is only about the length of a book, without the tail. She was the runt of the litter, yet she was the only kitten that survived out of her litter. My male cat is well beyond the age of ual maturity, and i'm afraid hes going to get her pregnant! What should i do? I cant afford to get her spayed...