Hi @stillzman,
As mentioned in this post-FAQ for the 45HomeLab HL15 - #71 by LostFate
the Max size motherboard in the HL15 is 10.5"X13". As long as your Motherboard is less than that you should be ok.
As for the cooling, I don’t have a solid answer for you on that because we do not have that CPU but the fans we chose for the HL15 allow a lot of airflow through the front of the system back to the motherboard which allows us to cool the full build 6 core CPU passively.
Now the EPYC CPUs are hotter, but you should be able to put a big air cooler on it to cool the CPU. there have been plenty of people on the forums getting Noctua NH-D9 DX-3647 4U coolers to fit although not the exact cooler you would use other similar coolers should be enough cooling for the EPYC CPU you plan to use