it will fit in a x16 slot running at x4 speed, but it is not recommended to run a graphics card in any slot other than a x16.
The specs are
1 x PCIe 3.0 x16 slot
1 x PCIe 2.0 x16 slot
PCI_E4 supports up to PCIe 2.0 x4 speed
2 x PCIe 2.0 x1 slots
3 x PCI slots
Will PCIe 2.0 x16 and R9 290 degrade performance on crossfire?