I have a 2018 Specialized Tarmac SL4 Sport and am looking to get a crank power meter. SRM or Quarq--still undecided on which one. Bottom brackets confused me. Do I just need to know the size of the spindle, and otherwise it'll fit? ie. 24mm vs 30mm? Or is there anything else to it?

Of course I need to know crank length, gearing, bolt spacing (BCD).

Thanks!