A tool to crop, add margins and change the media size is in the ToDo list, and for sure it will be able to do this operation. But I still need to code it.
With one of my other tools,
the PDFrizator, you may load the PDF and change the page size to some standard sizes. It only reduces to fit, i.e. if the content is larger than the size selected, but that is exactly what you need.
Another workaround is to print it to PDF, using the fit to paper size option and choosing the needed, or create a custom, paper size in the print to PDF printer settings. This is easy to do with the Adobe PDF printer that ships with Acrobat pro.
The PDF reader PDF-XChange Viewer export to image functionality has a page zoom parameter you may use to reduce the created image size, and so you can achieve your image size goal directly from the PDF to JPEG conversion.