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As @Ves says, the setting for this is in the DDJ-800 Setting Utility:

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As the title says I have a DDJ-800 that I use with the latest version of Rekordbox on a high end PC running windows 10.
The issue is I cannot change the buffer size, if I set it to 192 within Rekordbox as soon as I plug the controller in it defaults back to 256 and the slider becomes greyed out.
Is there any way I can change this? thanks in advance.
As @Ves says, the setting for this is in the DDJ-800 Setting Utility:

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Been a week and still no better off, thanks for the help guys much appreciated 👍...
Go to the settings utility of the device and check if you're able to change it from there.
That's what I have to do with my DJM-S11.