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In the context of stellar bow shock, the magnetopause is the point where the magnetic field is strong enough to brake the incident wind. The condition is that the magnetic pressure at that point is equal to the ram pressure of the wind

 B_m^2 \approx \rho v^2

If the planet can be considered as a magnetic dipole, then

 B_m \approx B_s \frac{r_s}{r_m}

where subscript s corresponds to the surface of the planet, and subscript m to the magnetopause. Putting it all together yields

 r_m \approx r_s \left( \frac{B_s^2}{\rho v^2} \right)^{1/6}

For example, earth's magnetic field is about 45 \mu T, the proton density of the solar wind is about 1.4 cm^{-3} and its bulk velocity is about  500 \frac{km}{s} substituting in the formula above yields

 r_m \approx 12.5 r_{\oplus}

where  r_{\oplus} is earth's radius.