I assume there must be a maximum size for insects, and I atmospheric oxygen probably has something to do with it. Insects do not breath like vertebrates, they have a series of tubes call trachea that run through their bodies and open to the surface at spiracles. Air passes through these tubes and oxygen and CO2 passively diffuse through the tissues. Insects can force air through the trachea to increase the diffusion rate, but they just aren't as efficient as vertebrates with lungs and hemoglobin to carry oxygen around the body.
In the past, oxygen levels were much higher. Higher amounts of oxygen could allow for larger invertebrates, and there were 6 foot long millipedes and giant dragonflies. Their size probably was not entirely caused by the extra oxygen, this was before they faced competition from vertebrates, so increased competition probably had more to do with their size reduction than loss of oxygen.
I don't know what the maximum size for an insect is at modern oxygen levels, but you'd have to eliminate the other selective pressures such as competition and limits on food before an insect could reach that size.