By utilizing a developed electromechanical coupling model considering the coupled flexoelectric and strain gradient effect, the dimension-dependent physical fields such as displacement, electric potential, electric displacement and carrier distribution were systematically investigated. It is shown that the flexoelectricity and strain gradient have a significant influence on the electrical related physical fields, but less effect on the mechanical displacement. Furthermore, in contrast to the strain gradient enhancing effect, the piezoelectric properties of micro-nano structures are suppressed by flexoelectricity. This study provides a theoretical guidance for analysis of the electromechanical characteristics of piezoelectric micro-nanostructures.