It's an interesting question. The Blue Whale is said to be the largest creature that ever lived on Earth. Weight isn't an issue since it's supported by its bouyancy. The largest land animal today is the elephant, but much larger creatures existed in the dinosaur era.

What factors might limit the size of living things, at least of animals? Perhaps one factor is the ability to consume enough food in its environment to feed itself. It's intresting that both the biggest animal (the Blue Whale) and the biggest fish (the Whale Shark) are filter feeders that consume huge quantities of plankton.

There could well be issues with the ability to conduct respiration at a rate sufficient to sustain itself. I remember reading that all cells must be in contact with a source of nutrients and oxygen.

For single celled organisms, one factor would probably be the ratio of interior volume to surface area. As an organism gets larger, interior volume grows at a much faster rate than surface area. This limits the ability to pass nutrients through the cell membrane.

Temperature could also come into play. As a rule of thumb, the rate of chemical reactions double with every 10 degrees Celsius increase in temperature. This might work against life evolving quickly in very cold environments.