Domain sizes in complex polymer materials on the 2- to 400-nm scale can be probed by 1H spin diffusion NMR with 13C detection, which may be competitive with microscopy. In glassy systems, two-dimensional 1H–13C heteronuclear correlation (HetCor) NMR with 1H spin diffusion is the method of choice. Limits to its applicability have been overcome here by improved data analysis. Single-spectrum referencing eliminates the need for asymptotic equilibration and expands the range of accessible domain sizes to long periods of ~ 400 nm and makes time-consuming measurements with series of mixing times unnecessary. Systematic 1H peak overlap correction in two-domain systems after local equilibration within 3 ms greatly expands the applicability of quantitative long-period determination from HetCor NMR with 1H spin diffusion. It usually works even if the 1H spectra of the two components are fully overlapped, as long as their fractional intensity contributions to at least one 1H peak are distinctly different. This is documented for microphase-separated diblock copolymers of polystyrene and PMMA (alkyl slices) and of polystyrene and poly(4-vinyl pyridine), a polystyrene analogue. Based on extensive spin diffusion simulations utilizing coarse graining to reduce simulation times, convenient graphs are presented that enable conversion of a measured equilibration percentage to a tight range of minimum and maximum long period, as a robust, model-independent result.