I set this out according to NZ Post Office guidelines even though we don’t use letter size. The addressee frame should expand downwards as it is filled
You can move the frames to where you want by dragging with a mouse or by using the Position and Size dialogue, Format - Image and shape - Position and Size I think (I don’t have computer in front of me). Frame Properties dialogue Format > Frame and Object > Properties > Position and Size
After the frames are positioned as you want you can save as a template. Click File - Template - Save as template, on the dialogue give it a name No 10 envelope and select My Templates as location, OK.
You can add mail merge fields to a new document created from your template if you need that.
More information on mail merge in the Writer Guide, download from English documentation | LibreOffice Documentation - LibreOffice User Guides