You probably mean our galaxy, The MilkyWay. First of all, it's very very big. When things get as big as our galaxy you can't measure them in kilometres. A unit called Light Year is used.
light year is the distance that light travels in one year.
Light travels at 2.997924562 x 108 meters per second.
year has 31,536,000 seconds, so, after some simplification
A light year is 9,463,000,000,000kilometres
Our galaxy is shaped like a saucer and it's diameter is believed to be about 100,000 light years across.
That means that if you had a spaceship that could travel at the speed of light, it would still take you 100,000 years to get from one side to the other.
There's a very good book called :NightWatch" by a Canadian astronomer, Terrence Dickinson, that has a chapter comparing sizes of things starting with the size of the Earth all the way up to the size of the Universe. See if you can find it at the public library.
Excerpted from Compton's Interactive Encyclopedia. Copyright (C) 1994, 1995 Compton's NewMedia, Inc.
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