Yahoo mail gives you a complete freedom to change the layout. infect, they have given some added great templates on the yahoo account. in case, If you are dealing with the yahoo account. you need to visit us for more help.
First of all, you need to launch the google chrome browser.
Click on the settings button and then click on the more settings.
Select the writting emails option and then click on the font style section.
Choose the font style and then select the size as well.
Once, you will make these changes, Your emails layout will be changed. for more details, you need to visit: how to change the font size on yahoo mail?