It appears to be Micro-ATX form factor. Often OEM motherboards use a proprietary front panel connector that is not the same as the front panel wiring of a retail boxed motherboard or retail bare PC cases.
http://www.ebay.co.uk/itm/Motherboard-LGA-1151-ACER-Predator-G3-710-MIB15L-SophiaB-FAULTY-/391633500351?hash=item5b2f2cf8bf:g:VIUAAOSw-0xYN251