You cannot make an image larger in Lightroom. It would be necessary for you to export a copy of the image and specify a larger size. You will have to determine whether or not the quality is acceptable.
An HD video frame is 1920 x 1080... or about 2 megapixels. An uncompressed file that size could be a couple of megabytes, but a compressed jpg could easily be much smaller than that, depending on the image content.
You cant predict the file size in Mb, you can only be assure the pixel dimensions will be exactly the size of the video.
thanks for your reply. The output files are indeed 1920X1080 but I am afraid about the file size, no more than 300Ko (I was expected almost 1Mo as you mention it), and about the image quality. Is there a way to specify a moderate compression for an output jpeg issue from a video ?
Is there any difference between the versions of Lightroom ?