The PEXP8-SX-8/4 is a PCI Express X4 lane convertor.
If your motherboard only has a PCI Express X4 slot, this adapter will allow you to install a PCI Express X8 in it. However your PCI Express X8 card will run at the X4 bus speed of 2000 MB/s.
The height of this adapter is 1.05"and the width is 2.94"
Other combinations of lane reduction adapters are possible with slight modifications, upon request.