The only thing more terrible than a terrible lizard would be a terrible lizard covered in feathers. That would be bad.
Imagine a 75 pound Chihuahua. Pesky little dog that becomes a nasty hyena like critter to everyone but its owner.
Aw, my giant frog will swallow y’alls cats, lizards, chickens, chihuahuas, and anything else you throw at him—and still have room for dessert! 
Many a D&D campaign will bear this out.
Ostrich then?And they can indeed be dangerous.
Why does these discussions always involve 50’s era SciFi monster sizes.
I think a 3 inch wasp would be pretty fucking scary, and may not violate the scaling problem. There are insects of about that size.
There’s a reason why things are the size they are. As mentioned, animals with an exoskeleton cant get too large or they would collapse. People don’t grow much taller than 6’6"/2m or so, and if they do they often suffer from life-shortening conditions.
Although some extant exoskeleton land critters get a bunch bigger than @Charlie_Tan’s mongo grasshopper.
See here for some recent Doperish exoskeleton-equipped drama:
Spoilered for potentially alarming visual once you actually go to the link. Expanding the spoiler is safe.
We caught an Amazonian Giant Centipede in our vacation house
A giant insect would also immediately suffocate because insects don’t have lungs. Their spiracle-based respiratory system won’t work when scaled up because it depends on oxygen diffusing directly into their tissues.
Years ago, I said to a friend who was a dog breeder, “Why don’t they cross pit bulls with something else? You could get a pit bull with the temperament of Cavalier King Charles Spaniel.”
He replied, “Or the reverse.”
Coconut crabs I think are pretty much the largest land based exo’s can go, and pretty terrifying. Scale up something like a scorpion to that size and that’s not going to be fun to deal with. Perhaps they can evolve some sort of carbon fiber nanostructure exo and get a good deal larger.
At that point, they are more vertebrates with a chitinous skin than an arthropod.
Not that that wouldn’t be terrifying.
yeah, shrews. There’s a reason why they are an epithet for a bad-tempered woman (because they are too weak to be dangerous but they want to be). Even a ten pounder would be snacking on your house cats.
Or “The Incredible Shrinking Man”.
Well, there you go!
For cryin’ out loud!
Quite right. In a ~20% oxygen atmosphere. But see below.
Back in the Carboniferous period when the atmospheric O2 level was 35-ish percent, there were scorpion-like land critters up to about 3 feet long with a 3+ foot front claw span. I’d rather not.
There may have been even larger creepy crawlies we just haven’t found fossils of yet.
A 25# weasel would be extremely dangerous. I don’t think a 40# wolverine would stand a chance against one. They would probably be able to outrun a cheetah.
Praying mantises certainly outperform their harmless looks with impressive skills. They’ll munch on your head while you complain about the indignity of getting your head consumed alive by a leaf looking twiggie thing.
The sulphur-crested cockatoo is a common urban bird in eastern Australia, and often eats building trimmings with its scary chomper beak. They have also learned how to open rubbish bins. They hang out in gangs and while they’re pretty benign but smart, so if they were bigger they’d probably start taking on cars and problem-solving their way into locked buildings as gangs.
If you’re having a quiet sandwich in the park and a big crowd of them wander over it can be very Hitchcockian. I’d not want to take on an angry one now, so if it was double the weight and heft, I’d run.
Basically “Garfield” next to an oversized lasagna dish.