This is not a bug but I'm note sure if we should leave it the way it is, try it yourself (without those code tags):
[size=7][size=7][size=7][url=http://www.elkarte.net]ElkArte 1.0 beta is out!![/url][/size][/size][/size]
:P
Wowsa ... that is a large font :D
The easiest way is of course to disallow size to be nested into another size.
Then... there may be another way that is "temporarily" disable a tag (for the moment if is going to be evaluated), send out the disabled version and then re-enable it... it may break things, isn't it? O:-)
I added a disallow param to the size stuff, should prevent it and render only the outermost size control ... will add it to a PR in process.
You always make things too easy... :P