Exactly this ^^^ you can also go by weight and feed 10%-15% of your snakes body weight which is easier to do with frozen feeders since they are usually labeled with there weight, at least at my local pet stores. Feeding a rodent as large as the largest part of your snake is easier than carrying a scale around with you but I've done it plenty of times haha. Your snake looks like it can take bigger than a mouse hopper, at least from the pic. Do you know what type and size it was eating before you bought it? Also do you know if it was being fed frozen/thawed, pre killed, or live? I would go for rats though since they come in bigger sizes than mice so you can always feed one rodent a meal instead of 3 or more once it's older. Also if your feeding mice and it was already on rats then it could be hard to get it back on rats.
Sent from my SAMSUNG-SM-G890A using Tapatalk