I am writing a short story set in a world inhabited by modern humans (Homo sapiens), bulky humans (inspired by Homo Neanderthalensis) and tiny humans (inspired by Homo Floresiensis). In response to comments, the height of H.F. was roughly 1m. I might make mine smaller for effect, but not below 0.75m
I will not attempt to make it hard science fiction and freely include various kinds of extinct species (such as dire wolf), only broadly minding their actual habitats and possibilities of coexistence. I will also drastically alter the continents (i.e. build a different planet).
Right at the start of the story, where I will only use Tiny Humans for quite some time, I want to make the reader notice their size. I don't want to place them alongside a homo sapiens. One option would be to compare to extant familiar animals, but I am afraid the presence of many exotic animals will simply make the reader suppose all animals are a different size. I absolutely don't want to use any of our units, because I always find such things utterly immersion breaking.
What would be some good formations/objects in nature to compare against (without presupposing deep scientific knowledge)?
Please excuse if my concept is still fledgling. It is only 1 month old.
EDIT: Many answers here have added invaluable information. They also made me realise, how devilishly complex this might be. Like myself, I expect many readers to have grown up with dino books. I will not introduce dinosaurs, but insects from their time. The reader might assume the atmospheric conditions that existed long before humans and led to huge animals and plants! On the other hand, he might be guided by the recent history of some actual plants to assume them much smaller (f.e. earliest watermelons are thought to have measured about 2 inches in diameter). Finally under these atmospheric conditions, even inanimate things like fires may behave differently (Naively I assume more oxygen implied higher flames).
In the beginning I thought the size of the full moon in the sky might help, but as you can't put anything at same distance, I see no easy way.
EDIT: Because the question of images received so much attention in the comments, I want to make clear that I see no difference between description and images. The painter, just like the narrator, needs to know what to put in the picture to convey scale. In the beginning I even thought some rare inanimate structure might be needed, which I would have to fit into my world!