What size should my SLI Bridge be?

Just ordered my second MSI Gaming Geforce GTX 970 4G GPU, but Have no clue what SLI bridge to use to connect the 2 cards on my MSI Z97 GAMING 5 (MS-7917) motherboard, I plan to put the cards in the PCI_E2 and PCI_E5 slots but I don't know the measurements.