Webflow has 4 viewports: the desktop,tablet, mobile landscape and the mobile portrait. But there are 3 more desktop viewports, the 1920px , 1440px and 1280px.
To add the 3 more desktop breakpoints click on the 3 vertical dots.
After clicking the dots choose what viewport you want to add on your designer.
A message box will appear when you click your selected breakpoint. Just click the create button to add the breakpoint.
Then a new viewport will be added at the top bar.
But if you want to view your work on larger viewports you can edit the canvas
Then in canvas settings put here the viewport that you want.
Changing the styles in different breakpoints is so easy in webflow just click at the top bar the breakpoint you want and pick an element to style.