The 29-year-old demonstrated how inconsistent clothes sizes are when she posted a picture of six of her clothes labels to Instagram earlier this week.
"Cleaning out my closet for my big sale this weekend, and couldn't help but laugh at everyone who says I lie about being a 22," she wrote in a comment alongside the post.
"Plus sizes are so inconsistent it's infuriating. In some brands I'm a 2x, in other's a 3x.. Sometimes a 4x! For my crop tops, they are mostly L from Forever 21.. That's a massive difference!"
She went on to say that you can't judge someone's size from their photos, nor should you.
"It only makes you look petty," she said. "At the end of the day, it's how you feel that matters, not the label in your clothes. I wouldn't care if the headlines said I was a size 0 or 28 model - it doesn't define me."
This is not the first time Holliday has spoken out about body image and acceptance.
In 2013 she started the #effyourbeautystandards hashtag, which encourages women to celebrate their bodies, whatever their shape. It now has more than 750,000 posts (and counting).
And last month, she teamed up with Simply Be for their #SimplyBekini campaign to encourage women to be more body confident this summer.
"I hope #SimplyBekini inspires women to love themselves and their wobbly bits," she commented at the time.
"All women need to do to get a bikini body is to put a bikini on, then they’re ready to hit the beach."