How big a DCP should be?

The maximum allowed bitrate in 2K or 4K DCP is 250Mbit/s or ~31.2MB/s.  This would mean that into a film in 2K 1.85:1 FLAT 1998x1080 the picture will use 243Mbit/s or ~30.3MB/s and the 5.1 surround sound will use 6.91Mbit/s (~0.9MB/s).

The final size of a DCP is related to the bitrate of the source file.

For example:
A film with a duration of 10 minutes 2K 1.85 FLAT, at 250Mbit/s, 5.1 sound, will have a file-size of about 18.2GB. With the same specifications, a film with a duration of 100 minutes will have a file size of 182GB. The same duration film in 4K would result in the same file-size DCP since the bitrate remains constant at 250Mbit/s and the only thing that changes is the compression factor. The actual final size of a film may differ since the compression of each frame is done in a Variable bitrate in JPEG 2000 codestream, not Constant bitrate as the examples above.

High bitrate SMPTE DCPs are still exmperimental.