It's Cold, Double the Heat

Simple

Power = $\frac{{V}^{2}}{R}$

Also $R=\frac{\rho l}{A}=\frac{\rho l}{\pi {r}^{2}}$

Power = $P= \frac{\pi {r}^{2}{V}^{2}}{\rho l}$

To double the heat you have to double the power hence simply double the radius as well as length because :

$=\frac { \pi { (2r) }^{ 2 }{ V }^{ 2 } }{ \rho (2l) } =2\frac { \pi { r }^{ 2 }{ V }^{ 2 } }{ \rho l } =2P$