When there is a definite article in front, you have to use weak inflection.
„Ein hoher Baum…“, but „Der hohe Baum…“
I keep referring people to this article cause it explained it so well for me that I haven't gotten it wrong since!: http://yourdailygerman.com/2012/10/08/adjective-declension-german/
read all three parts
With the possibility of misinterpreting your usage of big: The “not about the height” big is usually translated with something like
kräftig (strong) for very muscular persons as well as overweight persons – positively connotated
dick (thick) for overweight persons
fett (fat) – condescending and negatively connotated
Maybe schwer (heavy) does also fit your use case.