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 It is the common motherboard interface for personal computers' graphics cards, hard drives, SSDs, Wi-Fi and Ethernet hardware connections 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 More recent revisions of the PCIe standard provide hardware support for I/O virtualization
PCI_Express - Wikipedia, the free encyclopedia