There are some guidelines you can use. Feed weekly in their first year and fortnightly there after. An appropriate sized meal is anywhere from 5 to 20% of body weight. Size wise, that should be a little in excess of the maximum diameter of the snake such that once it has eaten it leaves a bulge about one and a quarter times the normal diameter.
The above is a conservative feeding regime designed to ensure your snake has sufficient food to achieve its full potential (not stunted) while at the same time minimising the likelihood of obesity or digestion difficulties. Well experienced keepers may feed items that cause the snake to bulge between two and three times its normal diameter. They understand how critical supplying adequate heat is following such a meal. They will also adjust the frequency of feeding accordingly. So don’t be surprised if you have people suggest heavier duty feeding schemes than the above.
Personally, I believe the most useful thing you can do is to learn to determine the body condition of your snakes. A snake in healthy body condition will have a small depression along either side of its backbone and will have a gentle taper of its body towards its tail. An overweight snake will look like a sausage in cross-section and have a thick body that sharply tapers towards the tail. The backbone is difficult to discern and the depression on either side will be rounded due to fat deposition. Scales are likely to be spread apart alerting you to the animal’s distension. In contrast, an underweight snake will have a backbone that clearly juts out, loose skin, will often feel quite hollow and in cross section will be almost triangular (a bit like a block of Toblerone chocolate, as some here mentioned. LOL.)
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