First I made an educated guess based on the tree size, which is roughly one and a half block. So I went for like, one and a quarter, this way the character wouldn't look huge while compared to a tree.
It looked decent enough, but felt that the overall vibe of the tileset asked for something a bit more cartoonish/SD, so went down 1/4 of a block. That version looked great already, matching the vibe, but I felt like trying at least one size down.
So I went down roughly 2px on each direction. It gave me a much simpler character that looked adorable and matched it nicely.