The biggest known star is VY Canis Majoris; a red hypergiant located about 5,000 light-years from Earth. Its size is estimated to be more than 2,100 times the size of the Sun:
Comparing it to our Solar System: Imagine the sun growing and swallowing Mercury, Venus, Earth, Mars, Jupiter and then Saturn. Well this how big this star is!!! Our observable universe (the one we can detect with today's equipment) consists roughly of 100 billion galaxies with each galaxy consisting of about 100 billion stars. This means that the number of observable stars is equal to 100bln x 100bln (that's one with 22 zeroes next to it). If you can count 20,000 stars per second you will need more than 13.7 billion years to finish counting!!! Stars start their life as huge spheres of Hydrogen gas, but a nuclear reaction at the core converts this Hydrogen to Helium, Carbon, Oxygen... then all elements up to Iron. Contrary to intuition, the bigger the star the faster it reaches its demise. Huge stars end their lives with great explosions called Supernovae. However, smaller stars (like our sun) end their lives with feeble ember called "White Dwarfs".