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Right click on the dtp in design mode. Click Format Control...
In the Size tab you can set the sizes. Also, in the Properties tab
you can set the control to:
  • Move and size with cells (will adjust control to cell's width as user resizes columns)
  • Move but dont size with cells (self explanitory)
  • Dont move or size with cells (self explanitory)
Check you setting to see which you have it set to.

HTH