I want to modify the font size in the content of the posts

I like a lot your theme, and I use it in mi blog ( https://www.oblogdeleo.wesped.es/ ), but the size of the letters in the content of the posts is a little big for me.

I would modify then, but I don’t have any idea of programing and wordpress, and I have no idea what I need to modify with the editor.

I hope you understand my horrible english