Standard sizes are:
468x60 Full Banner
120x60 Button
120x600 Skyscraper
160x600 Wide Skyscraper
120x240 Vertical Banner
125x125 Square Button
234x60 Half Banner
300x250 Medium Rectangle
468x60 is most commonly used.
You should try to stick to a standard size if you plan on distributing it for other sites to use (affiliate programs, etc)