  We consider a spherically symmetric black hole metric in (3+1)-dimensions in
presence of a positive cosmological constant $\Lambda$. We use a general
approach as proposed in \cite{1} to transform the metric in co-moving
coordinates. Then for a general metric we have evaluated the angular shadow
size both in absence and presence of plasma. By using the aberration relation,
we then find the corresponding shadow radius as viewed by a co-moving observer
for Schwarzschild de-Sitter or Kottler spacetime. The effect of cosmological
constant $\Lambda$ and plasma parameter $k$ on the angular size of black hole
shadow has been observed in detail. Finally, by using the observed angular size
of M87$^*$ and Sgr A$^*$ black hole, we constrain the value of the plasma
parameter $k$ with a specific observational value of the cosmological constant
$\Lambda$ (or the Hubble constant $H_0$).
