- How much can the being eat before it is out of food?
So it looks like you have a very dangerous slime bacteria. Unfortunately, the blob's existence would not necessarily lead to mass extinction. Assuming these blobs can starve, they will likely fall into a predator/prey cycle.
As the population of prey begins to falter, so does the blob's ability to find food. They then begin to die off and the prey population rises again only to repeat the process.
But what if they are apex predators.
It is possible that, since they eat organic matter, no digestive system could completely dispose of them. Following the next month or so, the blob eats the eater, quickly teaching all species to steer clear.
- How big would the being be after it is out of food?
Wikipedia ballparks earth's total biomass at 560 billion tons C*. If the blobs are able to consume all life, given a rough estimate of bacterial volume/weight (0.6um3/picogram) lands a rough mass at.
The volume of the Mediterranean Sea (I am skeptical about this answer and open to revisions)
- Is it possible for this being to leave earth?
Almost definitely not. The sheer amount of energy required to lift an object of this magnitude would be far out of its capabilities. The only plausible escape of gravity would be partial, meaning it would be trivial to fling small pieces of the blob out of the planet's gravity well provided the blob is intelligent enough to even do that. These projectiles would be unguided and not likely land anywhere suitable to consume or reproduce.
- How big would it be before it is unable to eat its own weight in food, due to the surface being too small compared with hours in the day and amount of food needs to be eaten?
This question becomes impossible to answer without knowing many other variables such as the rate of consumption, mean time between food discovery, speed of blob, and specific details about the local ecology. The best bet is to maximize surface area by stretching into a pancake like object for the highest food collection rate.
*C stands for organically bound carbon or carbon being used in a living system.