The 2017 Cheltenham Gold Cup winner has not raced since disappointing at Leopardstown last Christmas, having won the John Durkan Chase on his previous appearance.
Harrington was forced to rule out a Cheltenham defence due to a fractured pelvis, but reports the eight-year-old to have performed well in his racecourse spin.
She said: “I was happy enough with him. He’s got to do a lot. We’ve a long way to go, but I was happy with him.
“Robbie (Power) was riding him.
“He’ll do more in the next 10 days.”
Sizing John is entered in both the Paddy’s Rewards Club ‘Sugar Daddy’ Chase over two miles and a furlong or the three-mile Savills Chase at Leopardstown later this month.