purchased yahoo mail app but unable to fit screen

can someone please tell me regarding the yahoo mail app l have just purchased for my mac computer how to increase the page to fit the full screen, l can make the letters bigger but the size is only a quarter of the screen and l find this hard to work with when l try tapping on view to edit to full screen as l usually do it just diverts back to the App. Tried to do a search on this and it informs me to tap the green button on the page but this button doesn't show on this page, your help will be much appreciated