A soap bubble is given negative charge, then its radius will be

If $\sigma$ is the surface tension and r is the radius of soap
bubble, then $P$_excess = 4$\sigma$ / r 
When the bubble is charged, $P$_excess =  $P$_{electrostatics}  $+\frac{4\sigma }{r}$

After electrification, surface tension decreases.
This decrease the pressure and increase the radius  $\left(P\propto \frac{1}{r}\right)$.