I am 5ft 7-8in and currently ride a bike with a 54 top tube measurement and have a 10cm stem and no setback on the seatpost.
I am undecided whether I should be going with a Small or Medium in these frames. I am leaning towards the Small since it would be a 53 top tube and I can run a longer stem that I currently do if needed. Please provide some input ! thanks
included are my measurements from the wrenchscience fit calculator which recommends the medium frame for me :
WS Recommended Sizes
Frame Size center-to-center: 53 cm
Frame Size center-to-top: 54 cm
Overall Reach: 66.86 cm
Saddle Height: 71.77 cm
Handlebar Width: 42 cm

wrench science

Your Measurements
Height: 67.50 in
Sternum Notch: 56.00 in
Inseam Length: 32.00 in
Arm Length: 25.50 in
Shoulder Width: 42.00 cm
Flexiblity: 4
Weight: 168.00 lbs
Foot Size: 11.00 USMens
Measurements.
Height
67.50 in
Sternum notch
56.00 in
Inseam length
32.00 in
Arm length
25.50 in
Shoulder width
42.00 cm
Flexibility
4
Weight
168.00 lbs
Shoe Size
11.00 USMens

A straight up post and a short stem is a prescreiption for a cramped fit.

You need to compare not only the TT length but the seat tube angle to figure the reach of the frame. A frame can have a 1cm shorter TT, but just as much reach as your current frame, if the STA is steeper.

A lot of your decision will depend on your handelbar height. The medium size frame will have a taller head tube, which is good if you can't tolerate much saddle to handlebar height difference.

FWIW, I'm 3-4cm shorter, with at least 1cm more leg length (73cm saddle height). I ride the 51cm LOOK 585 with a 25mm setbackpost and a 110mm stem. The handlebar height, measured from the floor to the top of the bars is about 87cm, with only a 5mm spacer and an 84 degree stem.

thanks c-40 i read some of your other posts and they are quite informative. here is some more info for you about my current ride:
HTA 73
STA 73.5

i was actually on a 12 cm stem previously and a shop put me on a 10cm to get me more upright due to not being super flexible.

I'm 5 7 1/2 and ride a 51cm 585 with 70cm saddle height and 35mm of setback. The nose of the saddle is 6cm behind the crank. I run a 120mm stem with 2.5cm of spacers. I'm not very flexible and run a 0 rise stem. On a 53cm frame I could run fewer headset spacers but also had to run a shorter stem which I didn't care for.

The steeper STA on the size S LOOK will produce about the same reach as your current frame. From there, you need to figure out the stem angle and spacer setup required to produce the handlebar height you need.

Shoot - I hope I didn't messed up. I just ordered a Medium 555 (53cm) and I am only 5'7" on a good day. I sold my small Giant TCR since I feel it's too small for me. I ordered the Look555 online so I did'nt even get to try one. Oh well.

T-E - you should be fine. No worries. I'm 5'5" and ride the small. I'm sure at 5'7", with regular build, a medium would be fine. Maybe you'll need to get a shorter stem, but shouldn't be an issue.