Think of a water pipe: making it longer makes it harder for water to flow (more resistance), and narrowing its diameter makes it significantly harder for water to pass through, especially since the area shrinks quadratically.

The resistance $R$ of a conductor is given by the formula $R = \rho \frac{l}{A}$, where $\rho$ is resistivity, $l$ is length, and $A$ is cross-sectional area. Since $A = \pi r^2$, the formula becomes $R = \rho \frac{l}{\pi r^2}$. By substituting $l' = 2l$ and $r' = \frac{r}{2}$, the new resistance $R'$ becomes $R' = \rho \frac{2l}{\pi (r/2)^2} = \rho \frac{2l}{\pi (r^2/4)} = 8 \left( \rho \frac{l}{\pi r^2} \right) = 8R$.

Always remember that resistance depends on the area ($\pi r^2$), so any change in radius is squared in the denominator. Changing diameter by a factor of $n$ changes area by $n^2$.