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Removing respiratory problems due to primitive lungs, is there any reason why giant insects can't exist? after all we have giraffes which are enormous but have incredibly skinny legs, quetzalcoatls too and horses ... and wolves and deer oh and terror birds... and gallimimus.
With adequate lungs would it be possible to have a preying mantis-like animal the size of a giraffe or maybe ants the size of modern racing horses?
You must do something with the circulatory system.
First, the haemolymph of insects does not have any oxygen carrying pigment, because it is not involved in transporting oxygen to the tissues. You must fix this.
Second, insects have what is called an "open" circulatory system, which means that their haemolymph is not confined in vessels -- it bathes and sloshes in the body cavity. This is not good for an oxygen-carrying function. You must give them a less open circulatory system and much more powerful hearts.
You must absolutely do something with their skeleton.
The exoskeleton of insects is not living tissue. It cannot grow. Insects have a complicated mechanism for growth. For an insect to grow, it does the following:
It accumulates nutrients and energy and raw materials.
It prepares a soft and stretchable version of the exoskeleton behind the old exoskeleton.
It sheds the old, too small, exoskeleton. At this point the insect is squishy and vulnerable. ($\leftarrow$ This is the big problem. After moulting and before the new exoskeleton solidifies, the insect has very low structural strength. That's the main reason they cannot grow very big.)
It then sets its metabolism into overdrive, growing as fast as possible while the newly exposed exoskeleton solidifies. The fixed size, growing-not-possible, stage between two moultings is called an "instar".
I don't know about "giraffe-sized", but during the carboniferous period (about 360-300 million years ago), arthropods were much larger.
The Meganeura were dragonflies that were about 4-5 times larger in wingspan than the largest extant species.
Pulmonoscorpius was a scorpion from that time period that was about three times longer than the largest extant scorpion species.
Eurypterids were a group of semi-aquatic arthropods that closely resemble modern arachnids (although they are more closely related to horseshoe crabs). Megarachne ("Big spider") was a spider-like Eurypterid with a legspan approximately twice that of the Goliath Birdeater Spider. While not all Eurypterids were large, Jaekelopterus grew to a massive 2.5 meters (8.5 feet)!
One of the main reasons arthropods used to be so much larger is that the atmosphere had a higher concentration of oxygen.
If you want to go with gills instead of lungs, you could have large inscet analogs on an alien planet who live in the water.
They would probably have to have a more advanced circulatory system like vertibrates have and having a lot more oxygen dissolved in the water than in modern seas would be very helpful.
If the planet's atmosphere has much more oxygen than Earth, there could be arthopods on land or in the air much bigger than modern insects, as there once were on Earth.
So if those giant insect like creatures are also vemonous like some insects, a human explorer (possiblyone with an nsect phobia) might jump into a body of water to secape from really killer bee like creatures. Only to find that the arthopods in the water can grow much larger, and that it is only the juveniles who crawl on land or fly in the air, to escape predators when they are young and small.
