Some people will say to feed them as much as they'll eat in a couple of minutes, which usually works fine, but axoltotls are opportunity eaters, which means they're really likely to overeat just because the food is available. The general rule is that the widest part of their bodies should be about the same width as their head, but that takes time to assess.
What kinds of food are you planning on using? Worms (such as canadian nightcrawlers) are generally regarded as the most nutritious food, but pellets with high enough protein are also fine, especially if alternated with worms. With an axolotl that small, you'd likely have to cut up a worm into smaller pieces. If I recall my own axolotl at a similar size, he reliably ate about half a nightcrawler (usually he'd go for the second half if I offered it, but sometimes not) a day OR about 8-10 very small pellets (mine are from Amazon, NOT the larger hikari sinking pellets). Things like bloodworms should only be a treat since they don't have a lot of nutritional value.
Now that mine is an adult, his metabolism has slowed down so I feed him one large worm about every other day or every two days depending on its size.
A lot of it will be trial and error to see how often your axolotl needs to be fed, but my advice would be to keep an eye on its weight and try to keep up at least some variety in food type. Also, make sure to clean up any uneaten food after the few minutes is over to prevent it from affecting the ammonia levels.