Fonts are very important — to start, you want people to have an easy time reading what you're trying to say. If they have trouble reading your font, you've lost them and they won't continue on — no matter how important the message.
Your font shouldn't overpower the message, yet it should be visually appealing and engaging. Have you ever looked at an article or advertisement and thought: that looks like work to read; so...I'm just going to skip it? That's exactly the type of situation we are trying to avoid. That said, it is ok to use various sized fonts, but stick to one or two within your brand identity — too many fonts and things start to resemble a ransom note (not quite the message you want to send)!
Size is important as well - be aware of the platform you're creating content for and be mindful of how small the font can be. Don't limit your audience by making the font too small, nobody should strain to see your message.
Questions about selecting fonts or building your brand identity? Ask us! firstname.lastname@example.org