We introduce a new technique to estimate the comet nuclear size frequency distribution (SFD) that combines a cometary activity model with a survey simulation and apply it to 150 long period comets (LPC) detected by the Pan-STARRS1 near-Earth object survey. The debiased LPC size-frequency distribution is in agreement with previous estimates for large comets with nuclear diameter ≳1 km but we measure a significant drop in the SFD slope for small objects with diameters <1 km and approaching only 100 m diameter. Large objects have a slope αbig = 0.72 ± 0.09(stat.) ± 0.15(sys.) while small objects behave as αsmall = 0.07 ± 0.03(stat.) ± 0.09(sys.) where the SFD is ∝10αHN and HN represents the cometary nuclear absolute magnitude. The total number of LPCs that are >1 km diameter and have perihelia q < 10 au is 0.46 ± 0.15 × 109 while there are only 2.4 ± 0.5(stat.) ± 2(sys.) × 109 objects with diameters >100 m due to the shallow slope of the SFD for diameters <1 km. We estimate that the total number of ‘potentially active’ objects with diameters ≥1 km in the Oort cloud, objects that would be defined as LPCs if their perihelia evolved to <10 au, is (1.5 ± 1) × 1012 with a combined mass of 1.3 ± 0.9 M⊕. The debiased LPC orbit distribution is broadly in agreement with expectations from contemporary dynamical models but there are discrepancies that could point towards a future ability to disentangle the relative importance of stellar perturbations and galactic tides in producing the LPC population. •we introduce a new technique to estimate the comet nuclear size frequency distribution (SFD) that combines a cometary activity model with a survey simulation•we applied the technique to 150 long period comets (LPC) detected by the Pan-STARRS1 near-Earth object survey•we find a relatively steep slope for the SFD of large LPCs with diameters >∼3 km and a shallow slope for smaller objects•we estimate that there are (0.46±0.15)×109 active LPCs that are >1 km diameter and have perihelia q<10 au•the total number of ‘active’ objects with diameters ≥1 km in the Oort cloud, objects that would be defined as LPCs if their perihelia evolved to <10 au, is (1.5±1)×1012 with a combined mass of 1.3±0.9 M⊕.