The difference between all those slots is how many pins, and thus how many lanes there are, and this is also the reason why some slots are shorter and longer than others.
So a PCIe x16 slot is the longest, also housing the highest number of pins. PCIe x1 is the shortest, also housing the lowest number of pins. However, you can have slots that are physically longer than the slot type they're wired for. For example, a PCIe x16 slot being wired for PCIe x4, or a PCIe x16 slot being wired for PCIe x8, the former being more common.
This is separate from PCIe generations, which have doubled bandwidth for every generational increase, using various technologies to support that increase.