Terri Irwin raises eyebrows with cheeky joke about how 'size matters'

The Irwins aren't exactly known for their risqué humour.

So fans were understandably surprised when 57-year-old matriarch Terri made a rather dirty joke on Twitter on Thursday.

She tweeted a photo of herself next to an enormous pizza to mark National Pizza Pie Day in the United States, and wrote: 'Size matters.'

While she was of course referring to the large pizza, the phrase 'size matters' is often used in reference to the size of a man's equipment. 

Terri's followers weren't expecting the wholesome zookeeper to post such a cheeky tweet, with one replying: 'In a pizza, agreed, Terri. Who knew you were naughty?'

Another fan wrote, 'It's a risky joke,' alongside a GIF of a man sweating profusely.

A third Twitter user with the suggestive name 'ben dover' responded: 'You're right. Size does matter.'

A fourth posted a well-known meme of a man expressing surprise. 

Aussie model Ellie Gonsalves, who recently got engaged at Australia Zoo, also tweeted a crying-with-laughter emoji.

It comes after Terri's granddaughter Grace Warrior met an echidna for the first time at Australia Zoo, under the watchful eye of her mother Bindi.

Grace, 10 months, sat in her mum's lap as she glanced at the spiny anteater less than a metre away, while Bindi carefully placed a hand on the animal's back.

'Happiness is meeting an echidna for the very first time,' Bindi wrote on Instagram.

'Sweet Grace Warrior loved watching all the digging for ants. Monotremes (egg-laying mammals) are the coolest.'