Depends on RAW data format. First, it depends on BAYER pixel depth (8, 10, 12, ...), second -- on encoding used to represent pixel stream. Considering a simple case of BAYER8 and BAYER10 (8 bit per pixel and 10 bit per pixel) with no compression, it would be 1 byte per pixel or 2 bytes per pixel accordingly. So for 5MPix picture with BAYER8 image size, a size of RAW data would be 5000000 bytes, around 4.76MiB.

However, it is only a raw data from sensor, it does not include any metadata associated with the image. If you would save RAW file as many cameras do, in TIFF container, metadata would probably occupy another 2-4KiB. Not a big number compared to raw data itself.

For BAYER10 without any compression it would double the size of raw data, i.e. 10000000 bytes, around 9.54MiB.

If any compression technique is used, file size generally would be smaller.