If you are dealing with rows where the height has not previously been
modified the height should be adjusted automatically based on font
size/single spaced.
Data imported from another file may prevent that from happening as it
should, but once the data is imported you can select the entire sheet by
clicking the gray box at the top of the Row #s (or just select the cells
containing the data), then change your font size or go to Format>Row>AutoFit.
Keep in mind that once you manually change row height the 'auto' aspect is
nullified.
You can do it with a simple macro to select a range/whole sheet & have it
execute the AutoFit command. Then put a button on a toolbar and assign the
macro to it and/or use a kybd shortcut.
Before you go to that trouble, however, you might try selecting the entire
sheet, applying the Rows/AutoFit command. In 2003 (perhaps earlier as well)
that resets the row height property to automatic. Perhaps that's all you need
to do.