something at or less than 14". girls tend to be less muscular (yea yea sexist blah blah) and they wont be wanting to shoulder some huge behemouth.
as a starting block https
/www.pcspecialist.co.uk/notebooks/ultraNoteIV-14/ £508. but thats just a placeholder so you got a general idea. but its close to 2KG which is already on the border of comfortable carry weight.
So best to look for something 14" and 1-1.5KG in weight. this is why people buy apples (light weight), but ofc pay for the glowing apple emblem.