Can you tell me how big a JPEG file I can create when converting a NEF file in Lightroom?
You said "how big a JPEG file", so you meant file size instead of image size.
Image size (dimensioned in pixels) will be same as the NEF, and 24 megapixels is 6000x4000 pixels (if 3:2).
The RGB data will be 3 bytes per pixel, or 3x24 = 72 million bytes. That is simply how large the RGB data is. An uncompressed TIF file will be that size (or very slightly more for overhead).
FWIW, the raw file is NOT 3 bytes per pixel. More like 1.5 bytes per pixel, before compression to be smaller. Raw is 12 bits (maybe 14 bits) per pixel, but only ONE color (one RGB channel) per pixel. Raw is 12 bits per pixel, but JPG is 24 bits per pixel.
The JPG compression will reduce the JPG file size. This is a variable with the JPG Quality selected, and in some degree with the editor program. Also the pictorial content can influnce file size (bland data compresses well, skies, walls, etc. Complex data does not compress as much, a tree full of leaves). There can be a 2 to 1 range of JPG file size just depending on the image content.
Mostly though, better JPG Quality is a larger JPG file (bytes). Within general limits, this JPG file size comes out maybe 1/12 of data size (smaller is over compressed, less good), or maybe 1/8 data size (reasonable normal decent value), or maybe about 1/4 data size (large and best quality).
These are vague facts... simply just do one and see for yourself.
The Nikon manuals give expected file sizes, which is a decent size, but not Maximum JPG file size possible. The Nikon D5200 manual (24 megabytes) says they do an average of 12 MB files. This is roughly 1/6 of the actual 72 million byte data size.