What size spanner M6
 

is there differant sizes?

Per Machinery's Handbook, ANSI metric hex head machine screws in M6x1 use a 10mm spanner.

Agreed that most M6 bolts use a 10mm box/open wrench. If the bolt has a hex socket head then the common hex wrench size is 4mm. Andy.

This might help, I've made a table:

Standard thread dimensions used with bicycles - Cycle Gremlin

For most it's a 10 mm wrench. For shifter cable anchors and some brake anchor bolts it's 9 mm wrench.

No, 6 millimeters is 6 millimeters. What is different is if it is a M6 socket head cap fastener you will need a different spanner than for an M6 hex headed fastener.

I should have also mentioned that there are brand/product specific fasteners which diverge from otherwise common spec. We see M5 bolts with all sorts of heads as a frequent example. So what wrench your bike's bolt needs is determined by the bolt and not some industry "standard". Andy