The high field properties of carbon nanotube (CNT) network thin film transistors (CN-TFTs) are important for their practical operation, and for understanding their reliability. Using a combination of experimental and computational techniques we show how the channel geometry (length LC and width WC) and network morphology (average CNT length Lt and alignment angle distribution θ) affect heat dissipation and high field breakdown in such devices. The results suggest that when WC ≥ Lt, the breakdown voltage remains independent of WC but varies linearly with LC. The breakdown power varies almost linearly with both WC and LC when WC Lt. We also find that the breakdown power is more susceptible to the variability in the network morphology compared to the breakdown voltage. The analysis offers new insight into the tunable heat dissipation and thermal reliability of CN-TFTs, which can be significantly improved through optimization of the network morphology and device geometry.