BMI

I'm new to this site, so please bear with me. I'm 57 year old with PFFD, who has successfully begun to lose weight by walkiing 3 miles every day. By all height/weight/ charts, I am 'morbidly obese; Can any medically-trained people estimated the ideal weight for a 3'10" female of large build? What exercises are 'safe' other than walking? Also. a visiting nurse told me in passing something re knowing my 'true height' by measuring my arm length. I beleve she called it "Vitruvian' chart. Can anyone explain this? Thanks
mary809@sbcglobal.net