The default row height will reflect either:
a) whatever row height you've fixed for the preceding row (if you've done that); or
b) whatever space is required to accommodate a one-line paragraph in the paragraph Style being used. Thus, if the Style uses a 10pt font and a 6pt 'before' spacing, the row height will be around 16pt;
plus whatever top/bottom border spacing you've set for the table.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]