Pueblan Milksnakes stay quite small! many people say a 15 gallon is just fine for them.
My Pueblan is almost a year old I think, and he lives in a 7 gallon rubbermaid I believe, and has room to spare still. They are a small snake. You could definitly keep a Pueblan in a 10-15 gallon for a year at least. They are also a shy species and would enjoy the smaller cozier space for the first year of life.