Why must the economy keep growing?  
I read in the news today that the UK economy grew only 0.6% in the
past quarter.  That seems pretty good to me. Why can't the economy
just stay at the size it is now?  What would happen if it did stay at
the same size?  It can't keep growing
forever (can it)?

Like so many things, the importance of an economic growth rate is
relative.  An economy can either grow, shrink, or stay the same size. 
A 0.6% growth rate might be fine provided that it is creating enough
employment for a growing population.  Economic growth leads to greater
output, which can lead to wealth, increased capital per worker, and a
higher standard of living.  On the other hand, economic decline,
recession, or depression can lead to increased unemployment and
poverty.  Flat "growth" might be sufficient if the number of people in
your labor force is declining.

If your economy "stays the same size" but, the number of people
entering the labor force continues to increase, you will have
increased unemployment.  Increased unemployment would tend to decrease
the purchasing power of the population.  At high enough levels,
unemployment puts wage pressure on those with jobs, in addition to the
lack of wages for those not working.  At some point, your industries
would need to avoid producing more goods than your consumers could
purchase, forcing these industries to shrink further, leading to even
more unemployment.

Economies can grow for extended periods of time, though the period of
"forever" is difficult to guarantee.  However, growth is not enough. 
It is important that growth be steady, or "sustainable", to insure
that growth does not lead to other economic hazards and disruptions.

I believe you will find the following information interesting:

University of California at Santa Cruz, Macroeconomics lecture
presentation entitled, "Why should we be concerned about growth?" by
Professor Mike Dooley.
http://econ.ucsc.edu/faculty/mpd/econ2/Notes/Lecture+12+Growth/sld003.htm
"When output grows faster or slower than the population average, the
standard of living will rise or fall."

Google Search Terms:  economies grow
://www.google.com/search?q=economies+grow

I hope that you find this information clear and easy to understand. 
However, if confusion remains, please do not hesitate to request a
clarification.

Thanks.  I've been wondering that for years.

Just an addition to snapanswer's great answer. 

The economy needs to keep growing since when it becomes stagnant and
the population and workforce grows, then there will be a decrease of
jobs as said above and also greater inflationary pressure since supply
will be low because of no increase in production. When there is higher
inflation, people will not be able to save their money in banks,
making the latter increase interest on swings to attract depositors.
When interest on savings increase fewer people will go to the stock
market and corporations and manufacturers will not be able to produce
more goods. The economy then tends to go down. Unemployment,
bankruptcies and business closures will then follow.

However; uncontrolled growth is not good either since when people are
more prosperous they tend to spend much more and also put inflationary
problems because of supply and demand. Then the irritatiing economic
cycle begins.

The Federal Reserve attempts to control such economic swings to the
best of their abilities. The link below would be a good stop as well.

U.S. Monetary Policy: An Introduction
http://www.frbsf.org/publications/federalreserve/monetary/

Thanks for the great question!

Easterangel-ga