How many residues can a reliable ab initio structural model contain?
Year on year reliable ab initio predictions are becoming more common for longer proteins. 400 residues is not an unfeasible length. Of course, loops still remain more of a challenge than helical and sheet secondary structures.

At CASP13, the Google DeepMind team used AlphaFold to produce a good quality model (T0594) of a 774 residue protein which is a WD40-repeat of RFWD3 (6CVZ). 
All of their CASP13 entries are here.