Materials in the nano-sized regime often show unique and size-dependent properties. Despite intense studies on nanomaterials a systematic structural and spectroscopic study on a given system and the associated size-dependent physical properties is rarely studied, in particular from nano-crystalline into poly-crystalline (bulk) material through a critical size-regime. We carefully investigate the crystallite size-dependent structural features of Bi2Fe4O9 ex-situ as well as in-situ. Temperature-dependent X-ray powder diffraction (XRPD) on an X-ray amorphous precursor of Bi2Fe4O9 stoichiometry revealed that both (Bi1-xFex)FeO3 and Bi2Fe4O9 simultaneously crystallize from 780 K on, and the perovskite phase transformed into Bi2Fe4O9 above 860 K. Thus, heating the precursor above this point for 2 h at a subsequent higher temperature produces pure Bi2Fe4O9 samples with greater average crystallite sizes. As such, twelve samples with different crystallite sizes from 35.3(4) nm to 401(17) nm were produced using the temperatures between 900 K and 1073 K. Rietveld refinements on X-ray powder data collected at ambient condition ex situ showed that the lattice parameters depend on the crystallite size; similar to the trend observed for the evaluated data of the in-situ XRD measurements. Significant changes of the lattice parameters are caused by strong distortions of the FeO4 tetrahedra and cooperative rotation of the FeO6 octahedra. Analyses of the widths and frequencies of the Raman modes support these structural features. The electronic bandgap and the 2nd absorption edge observed at around 700 nm are found to be a function of the crystallite size. Why the Urbach energy steeply drops down from 0.33 eV to 0.19 eV at the crystallite size of 64 nm leaves an open question, which is the critical length of the magnetic cycloidal spiral of the multiferroic BiFeO31.