Tell her she looks “not bad“ or “You look fine” as if she’s acceptable to you but nothing special.
The hotter the girl, the more important it is to be ambivalent about her looks. My girl is drop-dead gorgeous, and I almost never say a word about her looks. All I’ll say is “you look nice” if she is wearing something I like.
I’ve floated a “pretty” comment past her in the past in order to experiment, and she responded in a very “ho hum” manner. The prettier they are, the more they know it and the less effective comments about looks are. In my relationship, I sometimes joke “After all, I’m the cute one in the relationship” and she loves that.
In your case, she is insecure about her looks big-time, so an overall unimpressed attitude about her looks mixed in with an occasional and rare compliment would be in order. You’ve got to play each one by ear and calibrate. A 5 or 6 needs a little ego stroking from time to time. 7 on up needs very little, and excessive compliments about her looks are a negative in terms of her valuation of you.
I know I’m going off on tangents here as you are just in the beginning phases of communication with this girl, but I’m just in the mood to lay out my thoughts and tactics regarding compliments and lack thereof.
The answer to your specific question is in the first sentence of this post. The rest is just guidelines for any beginners who happen by here and can benefit from the thoughts of an older war horse.