  The dispersion measure (DM) of high-redshift $(z \gtrsim 6)$ transient
objects such as Fast Radio Bursts can be a powerful tool to probe the
intergalactic medium during the Epoch of Reionization. In this paper, we study
the variance of the DMs of objects with the same redshift as a potential probe
of the size distribution of ionized bubbles. We calculate the DM variance with
a simple model with randomly-distributed spherical bubbles. It is found that
the DM variance reflects the characteristics of the probability distribution of
the bubble size. We find the variance can be measured precisely enough to
obtain the information on the typical size with a few hundred sources at a
single redshift.
