  The 3090FE is 12.3 inches long.  It will fit in the R13 without the front fan shroud that comes installed with the R13.  The EVGA 3080ti FTW3 Ultra is 11.8 inches long and there is just about 4inches of space from the end of the card to the front case fan.  
So... My Precision 7920 was giving me issues. Jumped online picked out the best R13 possible, was ready to hit buy, and saw backordered due to 3090 till end of April. I couldn't wait. So I went with the next best GPU in terms of Vram the 6900XT. No problem, 2 week shipping time and arrives this Wednesday 2-23-22.  My current system rocks a 3080 Ti. I have a 3090 Founders Edition I wanted to put into my Precision 7920 but it doesn't fit and even bypassing everything to make it fit - it wouldn't post (though it is on the supported list in the correct slot with correct PSU and everything). Will try things to make that work after the R13 is in place and I can have the 7920 out of commission for a while while toying with the 3090 in it to make it work. Moving on - the 3090 for the R13 appears to be a dell specific form factor. I fear the 3090 FE I have will likely not fit/work?! Okay fine. Ill be okay till end of April to get a 3090... so I called Dell today and was told No, you have to buy the R13 with the 3090. There is no waiting list and it will forever be backordered. There is no dell part number available because it is not available to purchase. You can return your R13 and order the 3090 model and wait till April-May. Well... that won't work... I have enough money to just keep buying and returning Alienware R13 computers to stay within return window until the 3090 R13 ships... Is that what Dell wants? 6 open box returns of R13s?  Can someone chime in on how to get the proprietary 3090 when it is available so I can upgrade my R13 to be as good as what I can afford? Thanks all!
Aurora R13, 3090 GPU upgrade