Several people in this forum have asked DxO to implement a full-screen preview of the PRIME noise reduction options, or at least make the preview loupe in the adjustment palette a lot larger. You can find one such request here, with the option to vote for it and/or the other similar requests that are mentioned in that discussion:
Hope that helps. I tend to export each of my images as soon as I’ve finished working on them, then look at the TIFF or JPEG I exported to see if the noise reduction is good. If it isn’t, it’s an easy matter to go back to the RAW image, change the setting, and export again. Those who do batch processing will be a lot more frustrated by this problem.