While Denon recommends a max 1TB SSD for the internal SATA slot, I decided to go for 2TB so I can make a complete, 100% copy of my music library onto it. (Yes, yes, I’m a hoarder, I know.) Went with this drive: https://www.amazon.com/gp/product/B07GBRG2G8/ - formatted exFat, copied my library over, then deleted some musical categories I know I would never DJ live - classical, experimental, metal, jazz, etc. The collection came down to less than 1TB on the P4 drive. This means that I’m utilizing less than 50% of the available storage on the SSD - why does this matter? Because 49% and lower storage utilization is ideal for optimum drive performance. Once you get over 50% utilization, then you start seeing diminishing returns on SSD speed (and even moreso for spinning drives). You may not notice it right at 51%, but you will the more you fill it up.
So the moral of the story is, while Denon recommends 1TB max drive size, I think a better way to look at it is to try and only use 50% or less of the drive you put in there. Seek and read times will be measurably faster. If you fill up that 1TB drive to 99%, you may encounter some seek lag and issues with browsing and analyzing on the fly (assuming you haven’t analyzed the file beforehand).
Anyways just my $0.02. It is supremely satisfying to have this much music available on the P4 combined with incredibly fast r/w speed.