How does Unitcell size affect adsorption enthalpy in infinite dilution?

[Z.GAO]

When we calculate the enthalpy of adsorption under infinite dilution conditions, we find that the enthalpy of adsorption varies greatly with different unit cells. So what is the basis of unit cell setup? When I set the unit cell to less than 2 times the truncation radius, the results file prompts a warning. But according to the manual, there is no cut-off radius setting, which is why?