What's the bandwidth and form factor for PCIe x1 x4 x8 and x16?

Solution 1:

Bandwidth:

legacy PCIe 1.x - 250MB/s per lane (16x == 16 lanes)
current PCIe 2.0 - 500MB/s per lane (thus 16x PCIe 2.0 is 8GB/s)
upcoming PCIe 3.0 - 1 GB/s per lane

Physical size (from Wiki):

The width of a PCIe connector is 8.8 mm, while the height is 11.25 mm, and the length is variable. The 'minor' half of the connector is 11.65 mm in length and contains 22 pins, while the length of the 'major' half is variable. The thickness of the card going into the connector is 1.8 mm.

Lanes | Pins Total | Pins in 'major' half | Total Length | Length of 'major' half 
------+------------+----------------------+--------------+-----------------------
x1    |  36        |  14                  | 25 mm        |  7.65 mm 
x4    |  64        |  42                  | 39 mm        | 21.65 mm 
x8    |  98        |  76                  | 56 mm        | 38.65 mm 
x16   | 164        | 142                  | 89 mm        | 71.65 mm