In my story, in which I want to rely on scientific logic as much as possible, I want a creature to evolve to become at least the size of a country and you can live in space. The means of evolution for this creature are Time and multiple generations, reaching 100,000 years, controlling plant life and its biology, and controlling single-celled life in a world similar to Earth’s environment, except that it is a hundred times larger with gravity twice greater than Earth’s. Be a completely biological creature I want to remain within the scope of science, without supernatural materials, to know how it evolved and shaped, and how to solve the problem of lunch, heat regulation, response, reproduction, and movement without turning into a motionless ball or a chemical mass.
If you want a 100,000 year old super-giant organism capable of evolution, consider it as a mosaic organism
A mosaic organism is a single organism with more than 1 set of DNA. When an organism like a mammal mutates within its own lifecycle, it typically results in cancer that kills the whole organism because all of our organs have to work in concert to keep us alive, but simpler organisms like trees often develop mutations and become 2 distinctly differing generic profiles occupying the same body that can live together. Witches broom, pink grape fruits, and dwarf alberta spruce are all examples of trees we have today that were graphed from another tree's "cancer".
While your giant organism may be 100,000 years old, it is so big (and hard to kill) that it experiences regular mutations within itself allowing it to evolve not from 1 generation to the next, but many many times within its own life. If its environment becomes so harsh that part of its body dies off, other genetic clusters expand, consuming the necrotic tissues and forming new healthy tissues that can survive better in the new environment.
So, even though it is one organism, it is evolving just as much as a collective ecosystem made up of many deferent asexual species.
As for size and shape...
If you want it to be this size and NOT crush into a motionless ball when on a planet with 2Gs... it has to be a giant pancake like organism. An organism, under gravity, can be any length and width without much consequence as long as its not monstrously tall, it will be fine.
Only after being ejected into space will large parts of it die off, and some of the luck cancer cells take over reforming the whole organism into a space capable form. Once in space it can take on a much less flat form since it only has its own gravity to contend with, which will be negligable.