If all matter in the observable universe were compressed into a single sphere, determining its size would depend on various factors, such as the density of matter and the assumptions made about the universe's composition. However, it is worth noting that attempting to calculate this precisely is beyond the current capabilities of scientific knowledge and technology.
Nevertheless, we can provide a rough estimation based on some assumptions. The observable universe is estimated to have a diameter of about 93 billion light-years. However, the matter in the universe is not uniformly distributed, and there are vast regions of empty space between galaxies and clusters. If we were to consider the average density of matter in the universe, which includes both visible matter and dark matter, and assume a roughly uniform distribution, we could attempt an estimate.
Assuming the observable universe is mostly composed of hydrogen, the most abundant element, and using its average density, we can calculate the volume of the observable universe. Multiplying this volume by the density of hydrogen would give us the total mass. Finally, using the known relationship between mass and size for a sphere, we can estimate the radius or diameter of the compressed matter.
However, it is important to note that this estimation is highly speculative, and the actual size would likely be much smaller due to the presence of vast regions of empty space and the fact that matter is not evenly distributed. Additionally, the universe's composition and density are not precisely known. Therefore, any estimation of the size of a hypothetical compressed universe should be taken with caution and is subject to significant uncertainties.