What is the size of the universe?

The universe is about 13.8 billion years old. So any light we see may be travelling for 13.8 billion years or less. The scientists call it as the 'observable universe'.
The distance to the edge of the observable universe is about 46 billion light years because the universe is expanding all of the time.
If you want more information you try reading more at https://phys.org/news/2015-10-big-universe.html#jCp

The distance between the Earth and the edge of the observable Universe is 46 billion light-years. In this connection, we should remember that the distance measured depends on the time. The diameter of the observable Universe is approximately 91 billion light-years. Here also we must note the word ''observable''. So, there are some areas (maybe very vast) which can not be observed with the available knowledge.

Considering that the whole universe has originated from a sphere with a big bang, I will give you the diameters of popular astronomical objects in our solar system as well as the Universe. You can have an idea how big the universe is.

Earth's Moon - 3500 Km

Mercury -4900 km

Mars - 6800 Km

Venus 12000 km

Earth - 130000 km

Kepler 22b (Believed to have Earth-like atmosphere in the constellation Cygnus) - 30,000 km

Neptune - 50,000 km

Uranus - 51,000 km

Saturn - 120,000 km

Jupiter - 140,000 km

Proxima Centauri (one of the three stars of closest star system, Alpha Centauri) - 200,000 km

The Sun - 1,400,000 km

Sirius A (Brightest star in the earth's night sky)- 2,500,000 km

Vega (fifth brightest star in the earth's night sky) - 3,800,000 km

Arcturus (fourth brightest star in earth's night sky) - 36,000,000 km

Rigel (blue supergiant and seventh brightest star in the night sky) - 97,000,000 km

Betelgeuse (9th brightest star in night sky but could be the first brightest if human eye can see all wavelengths of radiation) - 1,300,000,000 km

Vy Canis Majoris (second largest star discovered so far with a size of 1420 solar radii)- 2,000,000,000 km

Uy Scuti (largest known star known so far with a size of 1708 solar radii) - 2,400,000,000

NGC 1277 (is a galaxy called as the relic of the early universe due to the stars in this galaxy formed during 100 million year interval about 12 billion years ago) - 60,000,000,000 km

TON 618 (extremely luminous nucleus of a galaxy, also contains supermassive black hole) - 400,000,000,000 km

Cat's Eye Nebula (first planetary nebula to be observed with a spectroscope by William Huggins) - 250,000,000,000,000 km

Helix Nebula (one of the closest to the Earth of all the bright planetary nebulae) - 3 Lightyears

Orion Nebula (brightest nebulae, and is visible to the naked eye in the night sky) - 24 light years

Omega Centauri (largest globular cluster in the Milky Way) - 150 light years

Small Magellanic Cloud (is a dwarf galaxy near the Milky Way) - 7000 light years

The Milky way (contains our solar system) - 100,000 light years

IC 1101 (largest galaxy known so far) - 6,000,000 light years

Bootes Void (Great Void, is a huge, spherical region of space that contains very few galaxies) - 250,000,000 light years

The Universe - 150,000,000,000 light years <

This is the rough estimate of the size of the universe but it can be many times larger than the above-mentioned dimensions.

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To calculate the size and dimension of anything physically, we have to be outside it. However as we are inside the universe we cannot calculate the size of Universe physically. Moreover when we stand outside it and measure it then it means that there is still space outside the Universe we measure.
Universe is taken as including everything. So to avoid going more and more confused, we have taken an assumption that Universe is "anaadhi-anantam" meaning having no beginning and no end, no boundaries, all pervasive, all inclusive.

What we now take into consideration is the limited space where the Earth, solar system and some identifiable stars and extra Solar system objects which can be noticed by scientific aids.

Our ancient Rishis have calculated and many things by their 'Jnaana Drishti" or their spiritual vision- many of which slowly the modern science also verifies and accepts as correct. The Vedas and connected works have some calculations and measurements on many such things.
For example the present 'Kali Yug ' has a span of 4,32,000 years and only about 5000 years is now over in this Kali Yug. Time and space go on repeating in cycles undergoing changes also.