I normally ride a 56 road frame with compact geometry. Should I drop down to 55 or a 53 for a track bike? Would it be best to err towards too big, or too small? Incidentally I am looking at the Pista Concept, my height is around 5'11/ 6'.
Thanks

Go with the 55. When in doubt buy the frame with the closest top tube dimension to your road bike. At 5'11" you shouldn't have to worry about standover clearance which would be the only reason to consider going with a smaller frame.