FSA Gossamer cranksets are for press fit (usually BB or PF30) bottom bracket standards. The Ultegra 6800 is optimised for 11 speed set ups, with Shimano Hollowtech 2 bottom brackets. If you want the Ultegra cranks and rings, you will have to get a press fit / Hollowtech BB adaptor first. I'm in the process of upgrading from Gossamer with a metal PF30 to Ultegra 6700 ( 10 speed) with adapted ceramic PF30 at the moment I've gone for 175mm crank arms, but I have got quite long legs, and my bike has a 57.5 cm frame. The Ultegra chainset is a much nicer bit of kit, but it's not the easiest of jobs for the unwary.