Text Sizer for YaBB 2.6.11
Version: 0.1 (0.1 alpha - First release - December 3, 2015)
 
This mod uses the text-size data in a user's saved 'postlayout' to change the default text size in Posts, IMs and Calendar items. This mod also allows the text-size to be set from the User's Control Panel.   

Instructions:

Apply the mod and upload (in ASCII mode):

cgi-bin/yabb2/Admin/Modlist.pm
cgi-bin/yabb2/Languages/English/Admin.lng
cgi-bin/yabb2/Languages/English/Profile.lng
cgi-bin/yabb2/Sources/Display.pm
cgi-bin/yabb2/Sources/EventCal.pm
cgi-bin/yabb2/Sources/InstantMessage.pm
cgi-bin/yabb2/Sources/Post.pm
cgi-bin/yabb2/Sources/PostBox.pm
cgi-bin/yabb2/Sources/Profile.pm
cgi-bin/yabb2/Sources/Recent.pm
cgi-bin/yabb2/Sources/Search.pm
cgi-bin/yabb2/Sources/Subs.pm
cgi-bin/yabb2/Templates/default/Calendar.template
cgi-bin/yabb2/Templates/default/Display.template
cgi-bin/yabb2/Templates/default/MyMessage.template
cgi-bin/yabb2/Templates/default/MyPosts.template
cgi-bin/yabb2/Templates/default/Post.template
cgi-bin/yabb2/Templates/default/Search.template
yabbfiles/ubbc.js
yabbfiles/Templates/Forum/default.css

Once installed: Go to AdminCenter -> Forum Settings -> Posting 
Set 'Minimum Font Height (in percent of base font-size of 12px)' to 55 ( or larger)
Set 'Maximum Font Height (in percent of base font-size of 12px)' to 600 (or less).

More on default.css: The mod makes the bare minimum changes to default.css for the text-sizer to work.
default.zip contains a copy of default.css with all the font-size attribute changes.

If you want to do it by hand:

To the body element css  add

    font-size: small; /* on standard browsers this should be equivalent to 12px type.*/ 

remove all:

font-size: 12px; 

find all:

font-size: 11px; 

replace with

font-size: 92% 

find all:

font-size: 10px; 

replace with

font-size: 83% 

remove font-size attribute from .newsfader.