I want to change fonts size and font family for my website.
Please help me. How can I do it without effecting other WordPress theme functions?
Open up the theme folder and edit style.css. Do a search for "font-size" and "font-family", but you'll need to understand which elements are being targeted.
You can use a Google Font plugin to select which fonts you want to use which part of the site. A lot of themes have this functionality built in, but it can easily be added.
Too look at the 600 or so fonts available, go to google.com/fonts. Some recommended ones that I like are Oswald, Lato, Raleway, and Lobster.