viewnx is definitely adequate for this, although i'm not sure if there's an easier way.
in general you want to reduce the size - i find that 640 or 720 pix on the long side is usually plenty. usually a quality setting of 7 or 70 (i forget which one nx uses) is more than sufficient, and it keeps the file sizes down. often as little as 5 (or 50) is plenty.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
Some email programs and servers have a size limit on attachments, so one may want a smaller an overall smaller file size. Resizing from a Jpeg may cause a loss of quality, so if one starts with the best quality image, the resizing and file format changes should cause the least amount of quality loss.
If the in camera is not the right size, then you will back to View Nx and using the save as Jpeg from a Nef allows one more control over the process than the camera. Also if one repeatedly edits a Jpeg, there is a real loss of quality and additional artifacts can be introduced.