You cannot effectively control the final output size on a user's system in all circumstances. The final size of a font on a user's display is dependent on the system's DPI setting, as well as the pixel density of the user's display, which commonly varies between about 85 and 150 ppi.
Thus, for example, 10 point font on a Macintosh (at 72 PPI) was represented with 10 pixels (i.e., 10 PPEm) whereas 10 point font on a Windows platform (at 96 PPI) using the same screen is represented with 13 pixels (i.e., Microsoft rounded 13.3333 to 13 pixels, or 13 PPEm). Likewise, 12 point font was represented with 12 pixels on a Macintosh, and 16 pixels on a Windows platform that used the same screen, and so on. The negative consequence of this standard is that with 96 PPI displays, there is no longer a 1-to-1 relationship between the font size in pixels and the printout size in points.
The user might also use some scaling functionality to counter e.g. bad eye sight.
In plain emails (e.g. no advertising material that's more image than text), your best bet is to send plain text emails and let the receiver decide how he wants those displayed.
You can always try to change the font size in the Format » Size menu, or change the defaults in Preferences » Composition » General » HTML for your HTML emails. (Plain text emails don't come with formatting)