Every document created in Pages starts with a document imported (then edited) or with a Template.
Open the template that you use for letters.
Without entering any text, select the header text and change its type size to the size you want.
Make any other format changes needed for that template.
Add any content that will be the same for ALL letters started from that template.
Then, before entering any content that is specific to the first letter you will write using that template, Save the open document as a Template.
You will be given the choice of adding it to the Template chooser or saving it to your computer.
The first choice will put the template in the My templates section of the chooser, where it can be selected like any other template in the chooser.
The second will ask for a location in which to save the template. You will be able to open the template by going to that location and double-clicking the file, or by choosing Open in Pages' File menu, then navigating to the file's location and selecting the file.
Regards,
Barry