I’d go with CEO. CEO is more globally accepted. I’ve had buyers in the US who didn’t know that a managing director (UK) is the same role.
Don’t worry about looking too small. That’s massively outweighed by the fact that the prospect is getting an email from an executive rather than a pesky salesperson.
If you were in banking or healthcare then things would be different. I don’t think anyone cares if you’re a small but capable team in software, consulting and niches like that.