Hi Shane,
It is okay to use the word “beautiful” to compliment a girl you have known for a couple of weeks.
It is not a turn off to the girl.
However, it tends to be received best if used while in the process of a fun and engaging conversation that she is clearly enjoying.
If you use the word after making her laugh or smile a number of times during a conversation, she would appreciate that you said it even more so than if you use the word out of the blue or at random.