Because it's your first ones and the fact that you didnt use any tutorials, I'll give you a 7/10 : o
The size for one..all of them are quite big haha. And well then there's color, sometimes it's good to have a siggy in monotone, but having one or two different colors could also make them better. The pop out effect on the render imo looks bad and there's better ways of blending the render in than just outer glow. And uhm, about the brushing, don't just try to fill up the canvas with brushes that look awesome, that usually makes the siggy look too cluttered and creates no contrast. The viewer will be looking all over the siggy instead of just maybe concentrating on the point you want them to focus on, your render. Some adjustments with brightness and contrast might help with the first and last one. Otherwise impressive and keep up the good work : )