Create a data map with the Data Mapping Wizzard.

1) Select Multiple Symbol then Next>
2) Select Add Demographics to the map and set United States Demographic Data then Next>
3) Select Population (2002), Divide by is "none" and Show Data by Metropolitian Area then Next>
4) Set number of ranges to 1, set the Range Type to Descrete equal ranges, pick a symbol and set the data range to be from zero (0) to ten million (10,000,000) then Finish

You will get a symbol for each Metropolitian Area which be definition are cities with population greater than or equal to 50,000.

There are other population values you may want to use in step 3. (i.e. population female only, population of adults only, population with university degrees only, on n on n on....)
If you want more details increase the number of data ranges in set 4 and you will get different coloured symbols for each range (i.e. say 3 types cities from 50,000 to 100,000 / 100,001 to 500,000 / 500,001 to 10,000,000)

Play with the settings and you'll amaze yourself with pretty maps.

Ciao,

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Calv1ns