Hey, I've been looking into this too for my Mac Pro 2019. You're right that the max GPU length is around 305mm which rules out most of the big cards.
The best option for 3D rendering that fits is the AMD Radeon RX 6900 XT reference model. It comes in at about 267mm so it fits comfortably. Some third party 6900 XT cards like the Gigabyte OC version also fit - I've seen people run those in the Mac Pro 2019 without issues.
A few things to keep in mind though:
- The Mac Pro 2019 only supports AMD GPUs, so no NVIDIA options unfortunately. The 7000 series cards are also not supported since there are no Mac drivers for them yet.
- For 3D rendering specifically, you want as much VRAM as you can get. The 6900 XT with 16GB is the sweet spot.
- Some cards might be too thick and block the next PCIe slot, so check the cooler height (dual height = 40mm is the max that plays nice)
Another option if you want even more power is to look into an eGPU setup over Thunderbolt 3. You could run a bigger card externally. But for internal, the 6900 XT is really the ceiling.
Definitely check the MacRumors forums too, they have a great thread listing all the specific card models that are confirmed to fit.
Hope this helps, good luck with the build!
