Looking for a tiny PC that still has space for a gaming-quality video card? SFF PC Cases is a remarkably detailed spreadsheet listing dozens of models, complete with cost, dimensions, volume and even important build tips. The very smallest are not practical for powerful builds, but the critical "Maximum GPU length" field is right there to help you out.
The gold-standard NCase M1 turns out to be only the 27th smallest case that can accommodate a GPU, and even the ultrawee Dan Case A4 doesn't hit the top ten! But it's also true that many on the list require fat external power bricks
(if you're happy with that, the Custom Mod and S4 Mini models are astoundingly tiny, though good luck finding them for sale) or impose other brutal compromises, such as proprietary power supplies, too-severe limitations on GPU size, or plain goofy design.
The smallest case that's widely-available, attractive, and (relatively) inexpensive? And not so small that assembly will be a nightmare? Probably the Fractal Node 202.