for frame comparos of size VS TT
check out the GEO specs on the bike sites
for example, Specialized Tarmac, TT tends a little 'over-square' to square (TT longer or equal to virtual ST) so a 58cm ST has a 58.2 TT
Cannondale Super-Six - slightly under square, so a 58 ST has a 57.5 TT

Orbea Orca or Onix - they do 57 cm! so even though they are 'square', their 57 cm ST comes with a 57 cm TT! a consideration!

other manus may have 57 cm with slightly under-square DIMS for TT... so you can get into the 56 cm range

and I would think that an appropriate 57cm would give a great 'fit' position as easily as a 58...

just some considerations and thinking inside a bigger 'box'...