3-D integration presents a path to higher performance, greater density, increased functionality and heterogeneous technology implementation. However, 3-D integration introduces many challenges for power and thermal integrity due to large switching currents, longer power delivery paths, and increased parasitics compared to 2-D integration. In this work, we provide an in-depth study of power and thermal issues while incorporating the physical design characteristics unique to 3-D integration. We provide a qualitative perspective of the power and thermal dissipation issues in 3-D and study the impact of Through Silicon Vias (TSVs) size for their mitigation. We investigate and discuss the design implications of power and thermal issues in the presence of decoupling capacitors, TSV/on-die/package parasitics, various resonance effects and power gating. Our study is based on a ten-tier system utilizing existing 3-D technology specifications. Based on detailed power distribution and heat dissipation models, we present a comprehensive analysis of TSV tapering for alleviating power and thermal integrity issues in 3-D ICs.