The font size inside a post can only be influenced by the heading.
Our eventteam made a summary of the working codes some time ago. Let me try to translate:
------------------snip---------------------

Headings

Six headlines are available:

[h1]Headline 1[/h1]

[h2]Headline 2[/h2]

[h3]Headline 3[/h3]

[h4]Headline 4[/h4]

[h5]Headline 5[/h5]

[h6]Headline 6[/h6]

Font format

Those are only visible via external webbrowser:
[b]bold[/b] [i]italic[/i] [u]underline[/u]

Paragraph

Not available at all.

Quotes

Quotes are working in game and external:

Links

Links are only available by external webbrowser:
http://www.ryzom.de/de/
[url]http://www.ryzom.de/de/[/url]
Ryzom
[url=http://www.ryzom.de/de/]Ryzom[/url]

Lists

Only unnumbered lists are available:

• [list][*]List item[/list]

Ignore tags

It is possible to ignore BB-Code tags completely:
[noparse][h1]Text with BB-Code[/h1][/noparse]

Colors

Except one color (indigo) all of the following are visible in the game browser. Hex or RGB colors are not possible.

[color=Black]---[/color]
[color=Sienna]---[/color]
[color=DarkOliveGreen]---[/color]
[color=DarkGreen]---[/color]
[color=DarkSlateBlue]---[/color]
[color=Navy]---[/color]
[color=Indigo]---[/color]
[color=DarkSlateGray]---[/color]
[color=DarkRed]---[/color]
[color=DarkOrange]---[/color]
[color=Olive]---[/color]
[color=Green]---[/color]
[color=Teal]---[/color]
[color=Blue]---[/color]
[color=SlateGray]---[/color]
[color=DimGray]---[/color]
[color=Red]---[/color]
[color=SandyBrown]---[/color]
[color=YellowGreen]---[/color]
[color=MediumTurquoise]---[/color]
[color=RoyalBlue]---[/color]
[color=Purple]---[/color]
[color=Gray]---[/color]
[color=Magenta]---[/color]
[color=Orange]---[/color]
[color=Yellow]---[/color]
[color=Lime]---[/color]
[color=Cyan]---[/color]
[color=DeepSkyBlue]---[/color]
[color=DarkOrchid]---[/color]
[color=Silver]---[/color]
[color=Pink]---[/color]
[color=Wheat]---[/color]
[color=LemonChiffon]---[/color]
[color=LightYellow]---[/color]
[color=PaleGreen]---[/color]
[color=LightGreen]---[/color]
[color=PaleTurquoise]---[/color]
[color=LightBlue]---[/color]
[color=Plum]---[/color]
[color=White]---[/color]

Combine codes

It is allowed to use several BB-Codes at the same time. However those are usually not displayed correctly inside the game and the order is relevant.

Works only externally:

[color=yellow][h5]-[/h5][/color]

Doesn't work at all:

[h5][color=yellow]-[/color][/h5]

(Comment by Nendra: Looks strange though. I think this was white when I first saw the post, now it is black from outside the game.)

Images

Images work internal and external. PNG and JPG are allowed but they should not be too large. There is no restriction regarding the aspect ratio. Transparent PNGs are supported as well.

[img]http://atys.ryzom.com/attachments/download/258[/ img]

Line breaks

Line breaks seem to look different from inside the game compared to an external webbrowser. Some BB-Codes force single line breaks, then it doesn't matter how many empty lines you insert afterwards.

Number of characters

The number of characters is very limited when using the editor in game. The external editor doesn't seem to have a restriction.
If a post is written externally it is not possible to edit it from within the game.

Greetins
Kyonlas & Sontje
------------------snip---------------------

Personal notes:

• There is/was a problem with extremely long threads that can crash the game but work for external browsers.
• The colors look very different when inside a quote.
• Most BB-Codes that are not interpreted by the ingame browser are written as plain text except unknown colors. They are displayed plain white without the tags.
• If the text of a topic is too long it disrupts the style of the thread in an external browser. However this doesn't change the design for the ingame browser.