A great explanation of the scaling problem for exoskeletal creatures (scroll to Vertebrates Are Giants; Arthropods Are Small)
In summary:
Exoskeletons have really terrible joint options, which are not good at bearing weight. Essentially, you can have a pin joint (think crab claw), or just soften your skeleton a bit and hope it holds together. Whereas invertebrates have many joint options, many of which are spectacularly load bearing (think about knees and hips). There's just not a way to replicate this with an exoskeleton.
Exoskeletons are really heavy. They make up a greater proportion of an animal's body weight than endoskeletons do, and this gets worse as you scale up (square-cube problem; cubing a larger number). This also means having a creature with both an endoskeleton and an exoskeleton is not going to work out well. They would weigh so much, they'd barely be able to move.
Speaking of moving, they're not going to be able to run without fracturing their leg exoskeletons.
Exoskeletons are expensive. Most large creatures with exoskeletons live in the sea. One of the main reasons for this is that they use biomineralization to "mine" the water for minerals to harden their exoskeleton. 671
Since their exoskeleton has to be molted and regrown, this is a lot more expensive than an endoskeleton which can be continually grown and not wasted.
Speaking of molting, molting is extremely dangerous for small creatures with an exoskeleton. Exoskeletons don't grow, so they have to drop their exoskeleton and live as a blob of soft goo while they expand and a new exoskeleton grows. For very small animals, this isn't too much of a problem; they've just lost their armor, which means they're tastier for predators. For larger arthropods, they have to find a safe place to do this, since they won't even be able to move (no support for their body means their muscles can't reliably move their limbs, let alone support the weight of their body). For gigantic exoskeletal creatures, molting is a deal-breaker. Imagine a human without a skeleton. You wouldn't be able to breathe or pump your blood. Same thing for a human-sized exoskeletal creature that's molting.
So... what's the maximum size of an exoskeletal creature?
Underwater: The largest American lobster weighed 44.4 pounds.
On land: The largest coconut crab weighed 9 pounds.
Those are your upper limits. 671