RadRichTextBox's document model supports single section per document, thus you cannot have pages with different sizes in one document. Multiple sections support is planned for the next official release - Q1 2012, expected around mid-February, and then your scenario will become possible.
You can create separate documents for the envelops and print them trough an instance of RadRichTextBox. If you need to create multiple documents from a list with recipients, you can use our mail merge
functionality (online demo is here
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