Hello,
I’m getting a retic and wanting to know how fast she will need a bigger feeder rat size, she’s currently on pups, so do I order rat pups and the next size, or just rat pups, it’s cheaper to order a little more due to it canceling out shipping costs, just wandering the time it should take for her to need the next size up in prey item
Trying to practice preparedness
It’s very difficult to gauge what feeders you need before the snake is in your care. They also grow at different rates depending on different factors which can include feeding amounts. At the very least, what I would do is order pups so it doesn’t change what the breeder has been feeding her. And order the next size up because as long as they stay frozen they’ll be fine for a while and you’ll have them ready when you need them.
At least for the first couple of week though I would try to keep her as close to how she was in the breeder’s care so you’ll be able to learn her individual habits and personality.
Retics can be offered a variety of prey items once you have her eating for you. They’re also little black holes when it comes to food. If you need to double up on pups because you have too many left, I’m sure she won’t mind. Or you can try offering something like chicken heart or a piece of chicken thigh meat as well.
I am not trying to be unkind here but if you are asking basic questions such as this one I am wondering if you have done your research/homework on these animals. I do not own a retic but these guys grow quickly into massive, intelligent animals that need to be controlled mentally and physically, not to mention housed proportionally to the their size and eventually fed prey huge prey items.
Again I mean no insult to you but I am just curious as to if you are prepared to keep and handle a giant snake that has some brains. Now that being said I am a bit jealous because a retic is my absolute out of the question dream animal that I will never have! 
. Btw welcome to the forum and I seriously wish you all the very best and please post pics when you get your baby!
P.S. If indeed you are prepared then please disregard my post! Sometimes I go a bit overboard! 
Depends on how old it is and how fast you want it to grow. If this is a mainland retic, you’ll be off rat pups and onto small rats within a few months if you’re feeding weekly. They will eat multiple prey items so you won’t have to waste anything if they outgrow the prey size. I literally feed mine in dog food bowls and just toss several rats in the bowl unless I have appropriately sized chickens on hand.
A mainland can easily be 5’ in the first year and eating medium rats if you feed it weekly. I feed weekly until they’re a year old but not huge meals. From 1 year to 3 years I feed every other week. After 3-4 years old I feed them once a month and usually only feed 10 or 11 months out of the year.