By default, Excel rows auto adapt their size to the font size. If you set the row height manually, it will remain fixed to this size until you set AutoFit (Menu->Format->Row->AutoFit) back. 

Read this property to know if the row has AutoFit or if it has a fixed size. Write to it to set the auto fit state. 

Note that by default FlexCel will not autofit the rows even if they are set to autofit. Excel will do it when you open the file. If you want to manually autofit the rows (for example to display them on a FlexCelGrid) you need to call AutoFitRow

TFlexCelImport Properties