Restrict the use of [size]

[buzzle]

Why not just use a custom MyCode?

Regular expression:

\[size=(?:\d+|xx-small|x-small|small|x-large|xx-large)\](.*?)\[/size\]

Replace with:

[size=medium]$1[/size]

...or simply:

$1

That will take anything other than medium or large and replace it with medium (or remove the size tags altogether of you use the second option.)