There would appear to be no clear-cut answer to this interesting question. Size is one obvious variable. Argentinosaurus is claimed to have been the largest land animal ever to have walked the Earth. With an estimated mass in excess of some fifty tonnes, it must have surely been the most massive. Possibly too, given its exceedingly well-balanced body plan, highly efficient birdlike lungs, and hollowed out bones, Argentinosaurus probably marks the limit to how big an animal can get while supporting itself against Earth’s gravitational pull (whales and other aquatic megafauna are another story, of course).
Meanwhile, weighing in at the other end of the scale, if barely at all, are the bacteria, and similar micro-organisms. In view of the advantages accrued from the surface area to volume ratio, it’s more than possible that these tiny critters could take quite a fierce pull without breaking out into a sweat. Are microbes able to live on the revolving drums of spin dryers, for instance? If so, we’re talking about a thousand plus gees. . .
And that’s just accounting for life down here on Earth. All the same, we are so magnificently ignorant about the existence of life elsewhere in the cosmos — and the conditions that could give rise to it — that a strong dose of humility is called for when trying to tackle the above question. Still, it’s worth bearing in mind that if life exists (say) in the atmospheres of gas giants like Jupiter, it could conceivably take the form of gigantic, hydrostatically adapted, whale-like organisms. The longish short story: ‘A Meeting with Medusa’ by Arthur C Clarke is a case in point.