I know that practially all other terrestrial invertebrates are inherently limited in size, primarily by their exoskeletons, and by their respiratory systems, which impose upper thresholds on their maximum sizes depending upon oxygen levels. But is the same true of air-breathing land molluscs? In the oceans, all of the largest, most massive invertebrates to have ever existed have all been molluscs. Other than competition, and predation, is there anything preventing the hypothetical evolution of gastropods the size of the very largest cephalods, like giant squids?
Other than competition, and predation
When you are on land you cannot rely on hydrostatic to hold your body. And with no skeleton (endo or exo that it be), I have an hard time imagining a large sized gastropod moving around without collapsing under its own weight.