XPS 435T/9000 Max Hard Drive Size ?

FWIW:  On my XPS 435T/9000, the PCIe x1 slots are "gimped" - they only run at a max of 2.5GT/s.  This translates to a theoretical maximum speed of about 250 MByte/s.  On the other hand, the x8 slot is capable of full PCIe 2.0 speed (5.0GT/s), so a x1 card in the x8 slot could go up to theoretical max of 500 MBytes/s. 

(To give you an idea of how the slot matters:  I have a USB 3 expansion card which in one of the x1 slots will only do 165 MByte/s, but in the x8 slot will do 220 MByte/s - the card's max.) 

I'll be looking to get a PCIe storage card for the 435T in the future for myself, and personally, I'll be looking for a x4 or x8 card.

EDIT, 12/23/17:  Since the 435T only has one PCIe 2.0, 5.0 GT/s slot (the x8) besides the x16 graphics card slot, all high-bandwidth expansion needs really need to be met with one PCIe card.  A SATA PCIe card would have limited usage (only internal storage and only SATA), while a high generation USB or Thunderbolt card would not. 

Current USB 3.1 Gen 2 cards at 10 Gbits/s with PCIe x4 or x8 can support full SATA III (6 Gbits/s) speeds through drive enclosures and compatibility with NVMe SSD enclosures.  (One needs to verify that a specific card supports full 10 Gbits/s with the 435T's PCIe 2.0 interface.)  Future USB 3.2 Gen 2x2 or Thunderbolt 3 PCIe cards would allow even higher speeds (20 Gbits/s and 40 Gbits/s).  When out, key will be whether these cards work at full speed with the 435T's older PCIe 2.0 interface on the 8x slot. 

Additionally, standardizing now on USB-C connectors allows compatibility between all the above mentioned formats, as well as allowing for higher power needs of drive enclosures more readily available through USB-C (USB-C power is typically up to 3A @ 5v or 15W as opposed 0.9A @ 5v or 4.5W for USB-A).