Background: In papillary and follicular thyroid cancers (PTC, FTC), nodal and distant metastasis are generally considered important determinants of recurrence and survival, respectively. However, there is no consensus about the threshold primary tumour size (PTS) for these determinants. The aim of this study was to assess size relationships for developing nodal, pulmonary, bone and overall distant metastases. Methods: This prospective study covered 139 (93 females and 46 males) consecutive biopsy proven patients with PTC (114/139, mean age $41.0{pm}15.7$ years, M: F, 35%:65%) and FTC (25/139, mean age $39.2{pm}14.3$ years, M: F: 24%:76%). Results: Average primary tumor size was $23.4{pm}11.1$ mm and $26.5{pm}13.1$ mm for PTC and FTC respectively (p value=0.223). Nodal metastasis was found more common in PTC than FTC (49% vs 28%, p value <0.05), whereas overall distant metastasis was approximately the same (13% and 24%, p value=0.277); however, bone metastasis was significantly higher in FTC than PTC (24% vs 5%, p value <0.05). Cumulative risk for nodal and distant metastases for FTC and PTC starts at PTS <20 mm and may indicate an unusual aggressive tumor behavior in the studied population. Highest cumulative risk for nodal and pulmonary metastases in PTC and for bone metastasis in FTC was found to be ${geq}50$ mm PTS. Conclusion: We conclude that a PTS of <20 mm may indicate an unusual aggressive tumor behavior with highest cumulative risk for nodal and pulmonary metastases in PTC and for bone metastasis in FTC with a cutoff of ${geq}50$ mm.