Answer Description
PCIe, or PCI-Express, comes in x1, x4, x8, x16, x32 speeds. The size of the expansion slot also varies depending on type used. While PCIe x32 is defined in the official standard, x16 is generally the largest you will find on the market.
Wikipedia
PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. PCIe has numerous improvements over the older standards, including higher maximum system bus throughput, lower I/O pin count and smaller physical footprint, better performance scaling for bus devices, a more detailed error detection and reporting mechanism (Advanced Error Reporting, AER), and native hot-swap functionality. …
PCI Express - Wikipedia, the free encyclopedia