Louisa is wearing jacket and trousers in size 3 (NB, scroll down to see Louisa in the size 3 tricolor suit which has slightly different grading) 

Height: 175cm/5’7.5" 
Chest: 95cm/37”
Waist at bellybutton: 87cm/34”
Hips around bottom: 112cm/44”

Louisa normally wears size 12, but sometimes takes trousers in a 14.

“I’m quite small on top. Trousers are difficult because I’ve got a small waist relative to my hips”