if your not looking for 12ft yet then i reccomend asking yourself this, are you looking for a docile relax snake or a handful fun to deal with snake? that would give us more direction and suggestions to make
my views on "starter reptiles" or "stepping stone" reptiles are they are a bad idea because they often get neglected or traded/sold once the real goal is aquired.. HOWEVER if you can honestly say you will treat them the same hen this is what i will suggest
if your looking for a calmer snake, go with a boa or blood python, both are calm and easy going snakes with proper handling and are more like a burm then a retic. so they are in the eyes of some "easier" to deal with
if you want a more curious/active snake like a retic in the end then i suggest a coastal carpet python, they are extreamly fun to handle, and are active/smart like a retic. their cages like mentioned do need to be taller, but even on a large scale a 4x3x3 is plenty for an adult of 10+ft which is extreamly rare!. my coastal girl will probly be around 8ft and will live out her life in a 72x24x48 more then likely because she is a climber so u put a radient heat panel and they instantly become a decoration because they are out.
think, take your time and read all you can and ask every question you think of no matter how small or stupid it may seeem people on this board are more then willing to help so you get exactly wat you want