I currently have an S340 with an ATX Mobo, a 240mm AIO, and a Mini GTX 1060 6gb. What would be the smallest possible case I can fit these parts in ?
Giving you this option as you didn't mentioned PSU.The thermaltake G3 is extremely small BUT you need a SFX power supply.
Just so you have an idea the S340 is 41.57 liters and the G3 is 23.58 liters.
If you really want an ATX PSU as well, the Raijintek Thetis is pretty small.
There's also some Silverstone Grandia cases that could fit an ATX motherboard at slightly over 20L in volume, but I believe they require modding to accommodate most 240mm AIOs, given how thick the radiators and fans tend to be.
The define C (37.95 liters) is very similar in size as the S340 (41.57 liters). Would make little sense....
If you put both side by side you will never notice that difference. I would imagine he wants to notice a difference, so in reality it would make no sense to change cases for one that is around 3 liters smaller unless its for another reason.