I did the same thing just about a month ago. little kitten runs out of the forest at me and would not take no for an answer. About the same size as that one, maybe a bit smaller actually. I am thinking two months old (and I have had cats for a lot of years!).. I take him to the $80 buys you everything (shots/neuter/worming/deflea) clinic and the vet tells me he is four and a half MONTHS old.
So that malnourishment can make em itty bitty!