Have you ever drawn a circle? That's a symbol. It might sound too simple and mundane to be magic, but that is already part of tradition.
Have you ever drawn a five-pointed star? That's another symbol, draw a circle around that and it's definitely used in Wicca a lot for identification, protection, connection, and for many meaningful reasons. It's called a pentagram.
All that said, I never really felt the magic in all this "sacred geometry" stuff. Galdhr, yes, because it's sound and that's just the way I feel the magic most easily is through song. But the symbols themselves? Not to say that they aren't effective, only that it's different for everyone. Lately I've been creating my own symbols, like I try to translate ideas from the astral plane into something physical, and a symbol is the easiest way to do that. Chaos magick uses something called a Monogram, which always looks different but that's the chaotic way of inventing new symbols.