  We use semi-analytical modelling of galaxy formation to predict the
redshift-size-evolution of elliptical galaxies. Using a simple model in which
relative sizes of elliptical galaxies of a given mass correlate with the
fraction of stars formed in a star burst during a major merger event, we are
able to reproduce the observed size redshif t evolution. The size evolution is
a result of the amount of cold gas available during the major merger. Mergers
at high redshifts are gas-rich and produce ellipticals with smaller sizes. In
particular we find a power-law relation between the sizes at different
redshifts, with the power-law index giving a measure of the relative amount of
dissipation during the mergers that lead to the formation of an elliptical. The
size evolution is found to be stronger for more massive galaxies as they
involve more gas at high redshifts when they form compared to less massive
ellipticals. Local ellipticals more massive than $5 \times 10^{11}$ M$_{\odot}$
will be approximately 4 times larger than their counterparts at $z=2$. Our
results indicate that the scatter in the size of similar massive present day
elliptical galaxies is a result of their formation epoch, with smaller
ellipticals being formed earlier.
