Do lizards expand in the summer and contract in the winter?
I think you might be over-extrapolating your knowledge a bit. Animals are mostly made of water, and water only changes density by about 1% between 0C and 37C, so you are overestimating the effects of expansion/contraction due to temperature (and most organisms don't experience such a wide range of core temperatures, outside of special circumstances).

Because water is such an important component of organisms, size would also change with variation in hydration levels (you can easily observe this in plants because many plants use water pressure to support themselves). I would expect this variation to be as great or greater than any expansion/contraction with temperature.

Also note that just because animals are cold-blooded doesn't mean they don't regulate their temperature. Cold-blooded animals move between sun and shade, water and land, etc to regulate their temperature externally.