Could I know the video size before I export it?
After I set the settings, before render it, can I know the video size?
The size of the video is set in the settings of the composition. In the render module of your render queue item it the size of the comp can be changed by the resolution. In the output module you also have the possibility of resizing the result or cropping it.
With your composition active you use Menu>Composition>Composition Settings to see the current settings of the comp.