zomg love your avatar. (I'm in the local RHPS shadow cast)
I don't know how "plus" you're wanting, I'm about a size 16-18, which is right in the middle between the med/large & small/medium Singer dress forms from Joann's...
I got the med/large because I figured better to make too-big clothing than too-small clothing.
This is from joann's website:
# Bust sizes 40''-46'' (99cm-114cm)
# Waist sizes 30''-37'' (76cm-93cm), Hip sizes 40''-47'' (102cm-117cm)
# Waist to neck length is 16'' to 19'' due to Exclusive torso height adjustment*
The waist-neck adjustment is CRUCIAL for me because all my height is contained in my torso... when I buy clothes they always ride up because I'm so long-bodied (and short legged... it's not fair)
I love my dressform. Her name is Big Bertha. Only problem is that with my odd porportions, I have to make her hips almost as wide as possible (although it's not my hips that are that large, but that mommy pudge I never got rid of) and her chest as small as possible.
I have made a dressform out of duct tape, it's EXACTLY my size, even though it's a pain in the butt to do.