TBH, i would recommend you look for a 280mm AIO for the 3900x or any of AMD's 105-watt CPUs. My 3900X can hit ~95 degrees with a Corsair H115i RGB Platinum AIO 290mm cooler when under load. If you have not decided on a motherboard, I recommend you research them because a number of X570 motherboards have proven to have inadequate power supplies (i.e. VRMs), notably all of the MSi MPG motherboards which can easily hit 100+ degrees which is way too hot.
I had a MSi X570 MPG Gaming Carbon Pro and that board sucked. The CPU would slow down once the VRMs hit 85-95 degrees (above recommended temps of 80 degrees). I replaced the board with an ASUS X570 ROG Crosshair VIII Hero board. The VRMs on that board only hit about 45-50 degrees with my 3900X, a lot cooler.