Hi,
well, the Sil1364 is a SDVO to DVI transmitter, the only output is DVI yes.
On the length of the card, that's in line with the PCIe standard, you can put a x4 card in a x16 slot, and it will figure out it can only use 4 serial links to aggregate the bandwith. IF the board has a physical x16 length, but only x4 pins on the pcb, well, it won't fit in a regular x4 slot. Most cases because the x16 slot has special functions, like swithcing between regular PCIe lanes to special purpose internal graphics links.
If you look further into the boards you mention in your link, you will find that these are specifically used for LGA775 Mainboards, and connect to the graphics signal from that chipset "ADD2 cards utilize the PCIe x16 port to receive SDVO signals from the North Bridge."
Essentially these boards do exactly the same what the Sil1364 boards do, they connect to the internal graphics function in the chipset. They are not a graphics adapter either. And specfic to that MB Northbridge chipset.
On the choice of mainboard, well, most (consumer) mainboards for the 13th Generation Intels feature a x16 and a x1 slot, sometimes in combination with a PCI slot. But, there are a number of MB's with a x16 and a x4, and i chose one.
As i will never use my scope wireless, and most of them that have wifi have the antenna-connectors near the top of the board where the interface for the MS250/MS500 is (which i use) and because i did not want to introduce RF signal internally in the scope, i decided not to pay the 35-40 euro's extra for something i did not want there.
I basically needed only two PCIe slots and one NVME interface. Ethernet, USB etc on the backplane, and it had to feature 2 dualport USB headers internally, to connect the front USB ports of the scope, and the touch interface.
Another reason to go for this board is that is uses DDR5 memory, as most Math functions in the scope are memory bound, the faster the memory, the faster the scope.
There are industrial boards available that have an LVDS interface , like the Asrock IMB-X1314 . But they go for 4x or more the price of a regular consumer Mainboard.
And i still have to figure out to get the lvds interface working, and get the cabling/connector etc...
The use of regular MB made more sense, certainly because i was not sure the 'old' Lecroy interface would work in a PCIe Gen5 mainboard.
So far the test cost me less then 400 Euro's (MB + CPU+16G RAM+512G NVMe), i took a change knowing if it wouldn't work i would use the parts building a small workstation i need.
But so far it works, just have to solve the LCD issue.
Bart