The original paper describing the Gibson Assembly® Method [Gibson et al., 2009] suggests the method can be used to assemble constructs in the size range of small bacterial genomes, and megabase assemblies have been demonstrated using this protocol. However, at IDT we have limited our gBlocks fragment assemblies to 2–2.3 kb gene constructions using 4–5 gBlocks Gene Fragments using the Gibson Assembly Master Mix, available from New England BioLabs. For larger assemblies, we recommend a sub-assembly strategy with sequence-verification of the component parts.
Reference: Gibson DG, Young L, et al. (2009). Enzymatic assembly of DNA molecules up to several hundred kilobases. Nature Methods 6 (5):343–345. doi:10.1038/nmeth.1318. PMID 19363495.