Well, if that's what astrobiologists say they might do better to talk to palaeontologists who have some insight into the mechanics of life.

"Animals of any size at all tend toward four limbs."

No, vertebrates do because they are all descended from a common ancestor (a creature similar to Pikia that had four legs. To see how things might have been it's worth reading up on The Burgess Shale, we could both be sitting here typing with our five eyes and weirdly segmented bodies and lots of legs ;-)

This applies to land animals, although the same thing tends to apply to aquatic ones.

For the reason quoted above - we are all descended from a common ancestor (although interestingly the number of digits at the end of those limbs was variable in the early stages of tetrapod development). This excludes the millions of insect species too who are supremely adapted to terrestrial life.

Bilateral symmetry is a major feature of animal life but it's not universal. Sponges are asymmetrical and many animals such as cnidarians display radial symmetry. As for the upper size limit of an exoskeleton, certainly in terrestrial and aquatic arthropods there have been bigger forms than now although I'm not sure of the reasons offhand (raised O2 levels might be involved?).

As for the Battersby quote, I quite agree. See the papers I suggested people look up in my earlier post regarding possible extraterrestrial cellular life exhibiting some interesting characteristics and it's always worth Googling alh84001 . . .

"The general consensus is that scientific laws are universal

Two plus two still equals four."

Absolutely, but that doesn't mean there is one path that all life must follow. There are endless chemical, environmental, physical variables that means chance plays a massive role in evolution. Of course aliens may exhibit convergent evolution with any number of Earth lifeforms, but chances are even they will exhibit some interesting and undreamt of variations.

I might be a bit crap at getting my point over. It's worth picking this book up to better understand the roles of chance and biological contingency in evolution - plus it's a fascinating read and everything in it applies equally to alien life as it does Earthbound:

Wonderful Life: Burgess Shale and the Nature of History by Stephen Jay Gould.