What size SLI bridge to take?

No, I doubt that will work. The GTX 1070 needs a double bridge (which that one is), but that board with its wide PCIe x16 slot spacing looks to need a 4-way length bridge. Not sure what MSI had in mind by not providing one or at least providing a link to one.
 
that mobo isn't SLI compatible anyways... if it was it would be in the title. believe me SLI is a big deal. also the bridge should come w/ the mobo size depending if its 2,3, or 4 way SLI. and if u lose the bridge, buy the 29.99 one fro nvidias website
 


The board is indeed SLI compatible. Read the specs (detail tab).
• Supports 2-Way NVIDIA® SLI™ Technology
• Supports 3-Way AMD® CrossFire™ Technology