By default, the text in the dropdown of ComboBox cell in GridDataBoundGrid control gets trimmed when the text has more letters. When you want to set the size of dropdown based on the text size, you can use TextRenderer.MeasureString() in the CurrentCellShowingDropDown event.
After applying the properties, the dropdown of ComboBox in GridDataBoundGrid is displayed in the following screenshot.
Figure 1: Output
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