You need to change the length of the song section - best set it to automatic, before you add the audio file.
See this help page (assuming you are using GarageBAnd 2 on an iPad - which GarageBAnd version are you using?):
http://help.apple.com/garageband/ipad/2.0/index.html#chs3c3ef5dc
- Open the song section controls and tap the section you want to change.
- Tap the up or down arrow next to Manual to lengthen or shorten the section incrementally by bars. You can swipe vertically to change it in larger values.
- If you want the section to match the length of your next recording (to the nearest bar), turn on Automatic before you record. This is especially useful when recording improvisations or longer parts. The Automatic option is available only for the last (rightmost) section in a song.
- Tap anywhere in Tracks view to close the song section controls.
When you shorten a section, any regions extending past the end of the section are shortened. When you lengthen a section, all regions that extend from the beginning to the end of the section now loop to the new end of the section. A section can be any number of bars, and the overall song can be up to 320 bars long.