These are completely proprietary in design -- the board will not fit a standard case, at least not without a major loss of features. Plan on replacing the board with a standard one if you need a larger case.  
I'm currently thinking about switching my current PC's components which are those inside of the Dell G5 5090 desktop into a larger case. But I'm unsure of what size case to get. From what I understand the case you get should be based on the size of your motherboard, and that's where I'm running into a problem... I don't know what size my motherboard is (I don't know the dimensions of the motherboard or whether it's something like an ATX or MATX board. The model of the motherboard is the "Dell G5 5090-0DXJD9 motherboard". It would be very helpful if someone could provide me with the dimensions of the motherboard.
Dell G5 5090-0DXJD9 motherboard