Just to make sure:
You do know that the regular Cooltek UMX1 can't take any sort of discrete GPU? And it's STILL quite large (160x345x300mm), and fairly expensive (110€). But, there's also the UMX1 PLUS, which does support GPUs, at least.
As for NCase M1, that's indeed more reasonably sized (250x160x338mm), WHILE it can also take a GPU, but it's also quite expensive ($185 + import&tax).
But then there's also the Raijintek Metis you might want to take a look at. It's fairly small (190x254x277), can take a GPU, and is dirt cheap (50€). I have the copycat of that, the Raidmax Atomic, which I'm fairly content with.
On hindsight, though, I would rather lose the GPU slot and a standard sized PSU and go WAY smaller, as I'm only using an AMD A10-7800 (APU), which has a fairly powerful integrated GPU. For this, something like Streacom FC8 Alpha, which can use the case itself as a passive cooler for the CPU/APU. Combined with a picoPSU, that would be quite nice. But they're also quite expensive. (150€ + 70€)