How It Happened (Maybe):
Astronomers have estimated the age of the universe to be approximately 13.8 billion years old. Because of the connection between distance and the speed of light, this means they can look at a region of space that lies 13.8 billion light-years away or 81,124,857,236,160,000,000,000 miles, not including the relativistic effects, which is a big word for taking into account the differences in time and space that occur in measuring two objects in motion and the effect that motion has on the result.
There are various theories of the structure of the universe from multiverse (many universes) to a membrane (sometimes referred to a brane universe, kinda like the slices of bread in a loaf of bread) and the one more popular is the megaverse or infinite universe.
The multiverse or many universes model, in general, is an attempt to explain away the incredibly huge odds against the theory the universe was a random chance occurrence. The multiverse theory stipulates that the universe is a repeating process of expansion and contraction or in other words a repeating big bang and big collapse.
The accepted view of the creation of the universe is called the Big Bang. While this name is a bit misleading and would be more accurately called the Big Expansion, it is generally believed that space the universe occupies instantly expanded, much faster than the speed of light while matter that makes up everything came behind it, thus abiding by relativity and not exceeding the speed of light.
As the universe began to expand, particles began coalescing to form Hydrogen, Helium, and other materials. Eventually, stars began forming as gasses collected and gravitational forces began pulling everything together.
Physicists can mathematically trace back the creation of the universe to just a hundredth of a billionth of a trillionth of a trillionth of a second in age, as all the material flowed from a single point or (singularity). When scientists try to go any further back in time they reach a wall where all physical laws and math break down and prevent any further calculations.
While physicists are able to trace our origin back to the brink of creation, there is still a single very simple question, "where did everything come from, that came from that single point in space?". Some speculate that it could be a collapsing black hole in another dimension but there is no evidence for that at all and is nothing more conjecture.
One rather large problem to the theory of star formation as proposed is that gasses began coalescing together and gravity pulled these gasses together to complete the star. Reality shows a much different outcome. Gasses in a vacuum expand rather than coalesce thus debunking this theory. Furthermore, with the trillions of stars within view, we still have yet to see evidence of star formation from this process.
It is currently believed that only 5 % of the universe is made up of material we can detect with approximately 95% of the remainder being dark matter and dark energy, which has yet to be understood. We cannot detect dark energy or dark matter with any methods we currently have and even the theory of dark energy and matter is subject to change.
The actual diameter of the universe is thought to be about 160 billion light years (1 light year - approximately 5.88 trillion miles), and of course, this is speculative but is the current best estimate. Since we now believe the universe's expansion is accelerating, it is impossible to know the real diameter, since it would be like trying to measure the distance between a runner and a car traveling 40 mph away from the runner, the distance keeps getting larger.
Physicists refer to a value called the Cosmological Constant which can be thought of as negative gravity. Since gravity is a super weak force, this force is extremely small as well. The precision of this value has to be between 1 part in a trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion, trillion or 1 to the negative 120 power. Do not even try to compare that number to anything, it just hurts your brain. Einstein used this factor to balance out the expansion of the universe and could be explained by dark matter and energy.
It is noteworthy that if this constant had been the smallest bit larger or smaller, the universe would not exist. This makes the chances of the universe just happening or evolving from chance, immensely improbable. If you consider the cosmological constant alone, it demands colossal faith to believe the universe just "happened". It requires less faith to believe a God (Creator) was the one who did it.
The Shape Of The Universe:
The size of the universe is dependent upon a great deal by its shape. Physicists have predicted the possibility that the universe might be closed like a sphere, infinite and negatively curved like a saddle or flat and infinite. It could also be shaped like a doughnut or a torus as well.
A finite universe has a fixed size that can be measured; this would be the case in a closed spherical universe. But an infinite universe has no size by definition.
A flat universe is an infinite universe; thus the size of the universe is infinite. Ultimately, we again do not currently know exactly what shape the universe is because there is no way to know with the current information available.
Since it is now known the universe is expanding at an ever increasing rate, and could continue to do so, finding a size could be forever impossible.
The Possible Shapes of the Universe