Size depends on your budget, your style, and honestly, your earlobes. Studs are measured by total carat weight (both earrings combined), so a "1 carat pair" means each earring is about 0.50 carats.
For everyday wear, 0.50 to 1 carat total looks balanced on most people. Under 0.50 carats reads subtle and delicate. Over 1.5 carats makes more of a statement. If you have smaller earlobes, bigger isn't always better—they can look overwhelming. Try before you buy if you can, or start smaller and upgrade later.