You are looking at DeRosa Avant full carbon frameset.
This was used for a growing girl, rode almost just two times before she out grew this. So you are getting a mint frameset.
Size marking on this one says 54cm.
Top tube actual is 55 cm, slightly sloping. Effective top tube(measuring parallel to ground) is 53.4 cm
Seat tube center of BB to very top of the seat tube is 54.5 cm
Head tube length is 15 cm

Headset is included. For still got plenty of room. This is classic 1 1/8" steerer tube top and bottom.
Bottom bracket is already installed to fit Shimano crankset. If you like to use SRAM GXP, you just need to fit an adaptor.
Seatpost will need a 31.6 mm size. (I also have this matching FSA seatpost for sale on other listing. I can bundle if you wish

Please feel free to ask questions via email or cell 818-321-2027