Are you using Custom CSS to set those font sizes because there is only an option to set the Main Font size in the Design Options?

I am not using custom css.
I use the Design Options to set the main font to Georgia 12.
Inside the post from the WYSIWYG editor i set the font and size to Georgia 16.
When i select Excerpt from Layout Options -> Frontpage i get the text from the posts appear as Georgia 12 (in the frontpage). Which is correct.
When i set Content instead of Excerpt the text again in the frontpage goes from 12 to 16.
(i wll make a screenshot to see what i mean)
My regards

Ok.
Here are the screenshots.
The theme's font settings:
http://dl.dropbox.com/u/13458986/general%20fonts.jpg
The theme's frontpage layout settings:
http://dl.dropbox.com/u/13458986/layout%20settings.jpg
I have configured the same category (lefttst) on both ,in order to "fetch" the same post.
You can see that in the right column the Teaser setting is set to "Content" and in the left is set to Excerpt.
The post as is written in Georgia 16.
http://dl.dropbox.com/u/13458986/post.jpg

The final result in the frontpage is this:
http://dl.dropbox.com/u/13458986/frontpage.jpg

You can see left column (with the excerpt setting) correctly shows the text in Georgia 12, while the right column ignores the theme's setting (Georgia 12) and fetches the font settings from the post (Georgia 16).

P.S.
To make it more clear i' ve changed only the post's font. Making it from Georgia 16 to Comic Sans 16
http://dl.dropbox.com/u/13458986/post2.jpg

The outcome in the front page is this:
http://dl.dropbox.com/u/13458986/frontpage2.jpg

That is because the excerpt strips all tags so it will remove the setting for the font but the content will display everything.

Is there a possibility if I select to display the content, to inherit the theme's font settings rather than the poet's settings?
I really need to display content but with the theme's font settings.
My regards

*post's settings and not poet's settings (T9 mistake)

You would need to use Custom CSS instead of the inline CSS.

This would only change the font size of your post content on the single post pages:

.single .storycontent { font-size: 16px; }

Thanx for your answer.
I managed to solve it with other way.
It seems that the "excerpt" is also affected by the <!--more--> tag.
So i increased the words in excerpt to 80 and switched the teaser to excerpt.
Using the <!--more--> tag i manually set the break where i want it to be.