Como descrever uma pessoa que não é gorda nem magra em inglês

Hello Floracastanho,

I suggest slim or slender.

That's it!

Also "she is not slim or fat, she is evenly proportioned/distributed". But "she is of average weight", as Donay has pointed out, would be more concise, easier to say.

The Guardian journalist wrote "today, however, I am not fat. Nor am I thin exactly.", good choice of words indeed.

Another way would be "I am not slim nor I am thin, I am of medium build" (meaning - average in height and weight). Anyway, such description has a flaw, it is a comparison with the average people of our time! And it varies from generation to generation.