I'm really struggling to decide what size bike my partner needs. I'm buying online, so she can't try it out in advance. I'm probably going to get her the Claud Butler Urban 300 hybrid, available in frame sizes 16", 18" and 20". She's 5'3" with inside leg 30" and most size guides suggest that the 18" frame would be right, but an email from Claud Butler advises 16". I've measured her for the ape test, and she's on the negative side - ie short arms, so perhaps I should go for the 16". Does anyone have any advice? I understand that frame sizes are variable between bike manufacturers anyway, so how can one ever tell?