Your request is a little vauge to me.  So - -

Let me try to be a bit clearer...

The OP asks for the included jpeg preview in a DNG file.

To change this setting you have to go to Bridge and under the menu Adobe Bridge CS(x) choose Camera Raw settings.

Look at the chapter DNG file handling and put a checkmark in front of the option 'update embedded jpeg previews. Then from the drop down menu choose full size

Or if you use Adobe Photodownloader to convert to DNG choose the settings next to convert to DNG and inhere set jpeg preview from medium to full size. (be aware the file size grows also with this setting)

Possible bug here?

Don't think so because the setting should stay sticky, it does on my system.

When having Photodownloader open choose settings next to the convert to DNG and inhere also change the preview to large.

If this also needs to be reset every time you should reset the preferences for Bridge.

 

Even default dng jpg preview size is in camera raw settings large, that is not in that photodownloader setting options. It remains to be medium. Every time using photodownloader, setting has to be change from sticky medium to large.

 

Don't quite understand that, does your camera shoot DNG as Raw format? Or do you shoot Raw and jpeg together as setting.

I think that Bridge preferences reset will be the next trick. Just need to find out how to do it

 

 

That is the easiest part, hold down option (mac) or control (windows) while restarting Bridge and from the menu choose reset preferences. This will put all your settings back to default.

If this does not help and you are on a Mac you can also delete the plist file for both Bridge and Photodownloader from the user library but I don't know the equivalent for Windows.