When choosing the right size enclosure for a pet snake, it's important to consider the snake's species and size when fully grown. A general rule of thumb is that the enclosure should be at least as long as the snake's full body length and as wide as the snake's body width. Additionally, the enclosure should provide enough space for the snake to fully stretch out and move around comfortably. It's also essential to ensure that the enclosure has proper ventilation, temperature gradients, and hiding spots for the snake's well-being.