Choosing correct sequence setting when importing a file that is set to 960x540 to Hi quality 1080



I have an mp4 that I'm importing to edit a Premiere project. The mp4 file is set to 960x540 but if I export the file out after I edit it from Premiere at 1920x1080 the image is small. I created the file that I'm importing into Premiere but I don't recall/know why it is set at 960x540. The frame rate is set a 23.976. When I finish editing the file, I want to post the file to social media and chose the High Quality HD 1080 setting to export. Is there advice on what is the smarted thing to do here? Thanks much for any feedback or suggested link.
