Self-assembly of nanopore-spanning lipid bilayers (npsLBs) paves the way toward chip-based integrated membrane protein biosensing. We present a novel approach to analyze the formation of npsLB at individual nanopores using quantitative analysis of high-resolution microscopy images. From this analysis we derive necessary conditions for the formation of npsLBs on nanopore arrays by liposome fusion and discuss the limitations of the process as a function of nanopore geometry, lipid membrane properties, and surface interaction. Most importantly, applying liposomes with diameters larger than the nanopore is demonstrated to be a necessary but not sufficient condition for npsLB formation. A theoretical model is used to discuss and explain this experimental finding.