What base image size to choose to assist easier downsizing?
There is no objectively correct answer, there are too many variables to consider such as the nature of the image you are compressing (how much entropy/how 'compressible'?), how much you would need to reduce the jpeg quality to achieve the desired filesize etc... If it is possible to use a lossless compression format then I would consider that as a solution.

If you want to take an empirical approach you can take some representative images, go through the process of converting them using your two methods, then compare the results. You can use a tool like compare for a qualitative metric or visual comparison.