Is this the same as in-camera JPEG?

Yes. The embedded preview is the JPEG the camera generated at capture time, processed with the camera's own engine including any in-camera picture style or colour profile settings.

How large are the extracted JPEGs?

Embedded JPEG resolution varies by camera model and settings. Most modern cameras embed full-resolution or near-full-resolution JPEGs. Check the file dimensions after extraction.

Images Tool Box

Utilities

RAW Thumbnail Extractor — Camera-Processed JPEG Previews

Every RAW file contains an embedded full-resolution JPEG written by the camera at the moment of capture. Extract it instantly — no RAW development needed. The result is identical to the camera's own JPEG rendering, retroactively giving you RAW+JPEG from a RAW-only shoot.