[NAME]    Dude, listen. You are not supposed to be using 100% of what the PSU is rated at. They are made to be most efficient at 50-60% of their load capacity. Red-lining it is the equivalent of running your car's gas pedal floored all the time. It also doesn't account for any power spikes as well. I recommend you take some time to watch YouTube videos on this and do a little more research.  In the screenshot you sent me, it literally says the absolute minimum recommended PSU is 511 Watts. Don't take my word for it, just look at your own picture. Furthermore, look at the recommended PSU size from the people who actually make this card. [URL] It says 650W Minimum. That's almost 200W over what we have. You could maybe get by with maybe a quality 550-600W for it, but that would depend on use and your other components. The PSU we have doesn't even have the two 8-pin connectors required to power it. Why, you ask? Simple answer: It's not meant to/capable of running it. Trust me, I've looked into it, just search the forum for my name and you'll see all of it. Do your homework.    To answer your original question again, 8-9 inches max should be okay inside this case, I wouldn't go for more than a dual fan layout inside this one.   
Hey guys, just a quick question about the specs of the Dell G5 for a GPU upgrade. What is the maximum GPU size that can fit this PC? I suppose that the original equipment would be the maximum size but due to technical ability insufficiencies I am unable to remove it to measure.
Dell G5 Gaming Desktop - GPU upgrades