I'm assuming an alternative Earth in the following. Same physics, climates, DNA based life, fish as common ancestor to all higher land based life. No centaurs or superintelligent insect colonies.
Maximum size is quite easy. Something like a gorilla. Much larger and falling over becomes a hazard to life. But an elephant doesn't have hands and can't evolve into a centauroid because it's body plan was set by its fish ancestor. I don't think a trunk is a sufficient substitute for hands for developing technology.
Now the interesting direction. Smaller. The basic physics is that surface area goes as length squared but volume as length cubed.
A creature 10% the weight of H.Sap. couldn't support a human sized brain. Is that necessary? I think not. There have been people with anomalously small brains which were discovered at post mortem of a "normal" person. And there's Alex the parrot who approached chimp intelligence on a walnut sized brain. So I'd deduce that a brain one tenth human weight might evolve to support human equivalent intelligence. One hundredth, I'd doubt. Not least because a third or more of our brain is dedicated to vision processing.
A half-scale human has one eighth of the brain volume. Such a biped would average around a meter tall. I've fudged upwards a bit because it would not need to be proportionally as thick set as a human. Smaller limbs can be more slender and a smaller body falls less hard.
Now given the necessary intellect and hands could it access technology? Consider some key developments.
Coming down out of the trees ( only environment that encourages evolution of hands?) Proto-humans could stand to see above the prairie. Is three feet high sufficient? There's no obvious route to using fire if you keep living in trees. OTOH meerkats exist.
Fire. Making fire by friction is not easy. The creature has to dump sufficient muscular energy into a dry stick in a short time window. Like most kids I have tried and failed. Could any creature with one tenth human average arm muscle mass succeed? Perhaps, especially if it had theropod dinosaur ancestry rather than mammalian. Birds are from that branch of the tree of life and can support a higher metabolic rate. Otherwise are natural fire sources sufficient to attain technology?
Weapons. A tribe of humans with spears can defeat any predator often enough that the predator learns to leave humans alone or becomes extinct. Could a one-tenth human-mass creature become the top predator? Also could they develop bows and arrows? A scaled down arrow rapidly becomes non lethal.
Agriculture. Could these creatures domesticate oxen and horses? If not then ploughing becomes problematic as does keeping large wild herbivores away from the crops. Also the wheel works better for ox-carts than llama-carts or human-carts. The Incas didn't get to the wheel.
I hope this is food for thought. Technology would clearly be harder to attain for a creature with human intellect but one tenth our body mass. It feels like a lower limit even if you bless it with dinosaur genes.