Absolutely ruddy fantastic job there, loving the weathering. :thumbsup
still haven't started mine.
i calculated the body width at 20" so quite close to your dimensions. If you are thinking its a little big i may bring it down to 18"w. my artoo has a body width of about 18.5" so that should be ok.
If you mark out your lines and score them with a blade, you can heat it with a heat gun and it will separate your lines and add better detail.