  In this work we are interested in estimating the size of a cavity D immersed
in a bounded domain \Omega, contained in R^d, d=2,3, filled with a viscous
fluid governed by the Stokes system, by means of velocity and Cauchy forces on
the external boundary of \Omega. More precisely, we establish some lower and
upper bounds in terms of the difference between the external measurements when
the obstacle is present and without the object. The proof of the result is
based on interior regularity results and quantitative estimates of unique
continuation for the solution of the Stokes system.
