"smallest" in terms of volume, is going to be expensive.
Something like the Lian-Li PC-07S is technically an HTPC case, but supports ATX boards. $400 though, and no RGB AFAIK.
https://pcpartpicker.com/product/fzNypg/lian-li-case-pco7s
Appears it does, albeit with a riser.....
As per "completed builds" on PCPartPicker:
I'd imagine airflow would be restricted with the GPU vs glass...... and short, custom PSU cables would likely be a great idea.
Can't tell definitively if the case includes the riser either.... but the website "specs" would suggest it does:
http://www.lian-li.com/en/dt-portfolio/pc-o7s/