This taller girl likes me, she makes it obvious, how do I let her down gently?

I'm 5'7 and she is 6'0 and she likes me, she has even hinted that we should hang out together alone without our friendship group, she constantly compliments me, flirts with me but I just don't want a girl that is around 5 inches taller than me. If she wears high heels then she could end up being 6,7 or 8 inches taller and that doesn't sit well with me, I find it emasculating, so how do I let her down gently and tell her I just want to be friends without hurting her, without revealing the real reason I don't want her and without impacting our friendship group?
This taller girl likes me, she makes it obvious, how do I let her down gently?
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