In the paragraph options, under Line spacing, you can choose "at least" or "exactly" and enter a number of points (e.g., 12 pt). Note that you can also choose "Multiple" and put in a decimal number, such as 1.25; you're not confined to whole-number multiples.
The question then is, what are you starting with? The default is "single," which is some number chosen by Microsoft as a reasonable leading, such as 12 pts for a 10-pt font.
However, this doesn't appear to be revealed in the usual places, though it is undoubtedly somewhere. Here is a link to some more info.
http://support.microsoft.com/kb/9211...ItMyselfAlways
Does this solve your problem?