Are you talking about volume here, or mass? If volume, then the biggest ones we know ARE indeed at the end of their "life", up at the top-right corner of the Hertzsprung-Russell diagram. To know the radius of a star, you need to know its distance, which there are different ways of working out, some more precise than others. In the case of VY Canis Majoris, one of the largest stars known, there's a very precise radio parallax distance measure from the VLBI. Once you know distance, how bright it appears to be, and the temperature (these second two are relatively straight-forward and easy to measure from Earth) you can work out how physically large the star has to be to produce the observed amount of light from the Stephan-Boltzmann law.

Do the brightness and temp give a clue to if the star is about to "die" or that it's just a large star?

Yes, we can put the stars in the diagram based on their brightness and their temperature.

If they are in the main diagonal, they burn core hydrogen and will last a while.

If they are brighter than the main diagonal, they burn shell hydrogen and either core or shell helium, they are close to their death.

If they are less bright than the main diagonal, they don't burn anything and are just slowly cooling down.

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Have we(humans) ever seen a star die?

Yes, each supernova is a star dying. Scientists see some with telescope every year. Humans have seen a few with just their eyes, SN 1604: Kepler's Supernova is the latest one in 1604.

Most stars die without supernovae, but we don't see those.

You don't count SN 1987A? From wikipedia, Ian Shelton "went outside to look with the naked eye, and saw that the bright light was indeed present".

I think you might be confusing size and mass.

A star (excluding systems with accretion) remains a relatively consistent mass for it's lifespan, but can increase in volume as it ages. This makes the older stars less dense and (for lack of a better term) more "wispy".

Stars with high masses will emit higher energy wavelengths closer to blue, while less dense red giants and supergiant's will emit more low engery wavelengths

I'm speaking size, not mass. I don't care about the brightness but only the diameter

Spectroscopy.

The surface temperature can be determined by the relative intensity of different spectral lines (the letter and number in the spectral class) or by the colour index (bluer is hotter).

If the distance is known independently, then given the apparent magnitude, distance, and surface temperature, the radius can be determined.

The width of the absorption lines in the star's spectrum (the Roman numberals in the spectral class) allows to infer the surface gravity of a star. And if you know the radius and the surface gravity you can calculate the mass.

https://en.wikipedia.org/wiki/Stellar_classification

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Are black holes considered stars? If they are, I was just reading a few days ago about the largest black hole/quaser/blazer ever found with a mass of 40 billion suns. How big or what is the size of a black hole of that mass can be? Is it bigger than the solar system?

No, black holes aren't considered stars. Those super massive ones are enormous, though - the radius is linear to mass, and a 1 solar mass black hole has a radius of 3 km. So a 40 billion solar mass black hole would have a radius of about 120 billion km! Which is about 20 times further out than Pluto is, on average.

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How does that compare to the further most reaches of the solar system? (Like, to the Heliopause? Is that the considered the edge of the solar system?)

It's about 6-7 times further out than the heliopause. (The heliopause is far from spherical, since the Sun is moving relative to the ISM, but from the close part of it.)