<p>In this paper, we present the first study of plane-strain problems within the framework of complete thermo-flexoelectric theory, incorporating strain-gradient elasticity, direct and converse flexoelectricity, as well as thermoelasticity. We derive the exact solutions for three typical thermoelastic plane strain problems, which are the mechanical-electrical-thermal coupling problem for an infinite-length strip, the mechanical-electrical-thermal coupling problem for a hollow cylinder, and the thermal eigenstrain problem for a cylindrical inclusion. We develop the mixed finite element framework for the plane-strain thermo-flexoelectric problems, benchmarked against the three analytical solutions. This study reveals that the electric field induced by inhomogeneous heating in thermo-flexoelectric solids exhibits a pronounced size effect. Notably, an increase in the strain-gradient length scale parameter diminishes the thermo-flexoelectric effects. This study not only deepens the understanding of the mechanisms of multiphysical fields coupling in thermo-flexoelectric solids, but also provides insights for designing nano thermo-electric converters based on the principle of thermo-flexoelectricity.</p>