How big could a living thing be?
Simply put is there an upper bound for the largest possible living creature in a scientifically rational world without incorporating anti-gravity, magic, the force, or other physical concepts we are not aware of?
In order for me to consider it living, it must respire (though not necessarily breathe), repair itself, grow/have grown in some way, and be able to react to stimuli in a way inanimate objects do not. If you have other suggestions as to what belongs there, I am willing to hear them, but I want to permit exotic creatures.
If a portion of the body does not have properties I would attribute to life, it does not count towards the size of the creature. For example, your hair does not count towards your size even if you have a REALLY big afro.
Intentionally I am not putting many restrictions. I do not care if it is terrestrial, mobile, reproductive, or sapient. All I care is that it is as big as possible but reasonably possible knowing what we do about physics, chemistry, and biology.
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This is only a partial answer (it only addresses land animals), but I don't know if anyone else is going to mention it, so I might as well.
Land animals rely on their bodies to support their weight. A lot of that support comes from their bones. Now, a bigger animal will have bigger bones, so the volume of one of them (I'll say the femur) must be larger. However, to support the added weight, the bone must also be thicker (i.e. greater in diameter). As animals get larger, the thickness of femurs keeps increasing until the thickness is unfeasibly large. Past this point, any animal could not have a femur of the correct size, and thus there is a weight limit (and thus size limit) to any land animal.
Note: This section was essentially a summary of Appendix A of my copy of Professor Walter Lewin's book, For the Love of Physics. It goes into more detail than I did here, but I think this paragraph should be sufficient to communicate this [minor, for the purposes of your question] point.
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