The real question is will her ghostlight be the same as her prior dick
My guess is it will fall somewhere in the range of 12-18", whereas Brint - judging by his scenes - was probably around 2+ feet.
The real, real question is will the PC have any input as to whether or not Brienne goes for a ghostlight? Like, will she just up and decide she wants one or will the PC have to gently nudge her towards what they want for her? I'm hoping for the latter as it seems to fit her character more, but eh whatever. I'm not interested either way.