2008 K2 Enemy size mediums measures out to a 54cm.
Will fit riders 5'7" - 5'11" tall.  Standover is just about perfect for a rider 5'9" tall.
This bike is lightweight and very responsive.  The Compact front chainrings are a nice spec with a 34/50T!
Product Specs:

• Frame: Butted 7005 Series Aluminum Cyclocross Frame, Reflex Road Tubing
• Fork: Carbon Blades, Aluminum Crown & Steerer
• Front Derailleur: FSA Energy
• Rear Derailleur: Shimano 105
• Shifters: Shimano 105 STI
• Cassette: Shimano 105 10sp Cassette, 11-25t
• Crankset: FSA Omega Compact MegaExo, Aluminum Rings (34/50t)
• Bottom Bracket: FSA MegaExo
• Chain: KMC DX-10SC w/ Removable Link
• Wheelset: Alex A-Class ALX-220
• Tires: Vittoria Cross XG Pro, (700 x 32c)
• Brakes: Kore Kross Race Cantilevers
• Brake Levers: Shimano 105 STI
• Headset: 1.125" Threadless, Integrated
• Stem: K2 Fit System (By: NVO), Cold-Forged Aluminum
• Handlebar: Kore Road Race
• Grips: K2 Weather-Resistant Bar Wrap (w/ Gel Padding)
• Seatpost Clamp: Alloy
• Seatpost: 2014-T6 Aluminum, 3D Forged, (w/ Micro-Adjust Clamp)
• Saddle: Selle San Marco Ponza K (w/ Kevlar Rear Panels)
• Pedals: Crank Brothers MXR Clipless