It is generally assumed that the hydrated electron occupies a quasi-spherical cavity surrounded by only a few water
molecules in its equilibrated state. However, in the very moment of its generation, before water has had time to respond
to the extra charge, it is expected to be significantly larger in size. According to a particle-in-a-box picture, the frequency
of its absorption spectrum is a sensitive measure of the initial size of the electronic wavefunction. Here, using transient
terahertz spectroscopy, we show that the excess electron initially absorbs in the far-infrared at a frequency for which
accompanying ab initio molecular dynamics simulations estimate an initial delocalization length of ~40 A. The electron
subsequently shrinks due to solvation and thereby leaves the terahertz observation window very quickly, within ~200 fs.