Welcome to the forum!
That’s going to be a lovely baby cardigan.
The back should be the same length as the left front which is 29cm for your size.
There’s a problem however but it’s easy to fix. The neck shaping is on the wrong side of the left front (blue arrow) and the left back (red arrow) shoulder.
It may help to put a safety pin or marker on the right side (RS) vs the WS of the cardigan. In the photo the RS rows are facing us. The left front neck shaping should begin on a WS row which would put it at the tip of the blue arrow. The left back cast off should be at the beginning of a RS row, near the center cast off.
It’s not so many rows to take out to put this right and well worth the effort.