The 22-month-old and his mother Shelley gather eggs from their flock of chickens every morning and Mrs Finlan said they were both taken aback at the example produced by Rosie.
“Even though Toby doesn’t have many words yet, when he saw this egg he said ‘wow’,” she added. “I think poor Rosie could have dislocated a hip laying it.
“We’ve given her lots of good salad and vegetables from the kitchen as a reward for her hard work.”
Rosie is one of the flock of 35 chickens which are kept at the family home in Ancient Lane, Hatfield Woodhouse, near Doncaster.
Mrs Finlan said she and her husband Martin, an engineer, recently won a prize for their eggs at the village show.
The massive double-yoke egg, which was measured at nine centimetres high (3.5in) by six centimteres (2,3in) wide, was cracked open on Wednesday.
It was used by the family for baking.
Although the egg laid by Rosie was large, it does not beat what is claimed to be the world’s largest ever egg, which was laid by a hen called Harriet in Essex, in 2010.
According to reports, her egg was around 23cms (9in) long and 11.5cms (4.5in) wide, beating the previous world record by a wide margin.
At the time her owner, taxi driver Tony Barbouti of Southend, said: “I think we may need bigger egg cups if they keep this up.”