In the css file can anyone tell me which item to target to do this ?

I changed the font size in these but to no effect:

.postedText{
font-family:Arial,Verdana, Georgia, Times;
font-size:12pt;
font-weight:bold;
line-height:15pt;
padding-top:10pt;
padding-bottom:10pt;
padding-right:5pt;
padding-left:5pt;
}

.postedTextCell{
border-right:1px solid #EBECE2;
}

If by the "posting text" you mean the text displayed in the posted message itself, then you've mentioned an exactly proper place to change the font styling. However if you need to change the text appearing when you type i.e. in the text area, look for the '.textForm' class.

If you still see no changes, clean the cache of your browser and let it to reload the new CSS; some browsers like for example Safari on MacOS could cache the styling for a very long time and there are some tricks to force them to reboot it.