The nine-year-old completed the rare treble of the Irish Gold Cup at Leopardstown, the blue riband at Cheltenham and the Punchestown Gold Cup in the first half of 2017.
However, although he made a flying start to the following campaign in the John Durkan Memorial Punchestown Chase, he disappointed at Leopardstown over the Christmas period and injuries have kept him off the track ever since.
While unsurprisingly cautious of planning too far ahead, Harrington reports Sizing John in rude health at present.
"Sizing John is good. He's hacking away and I thought he'd tightened up a good bit," said the Moone handler.
"We've not made any plans, but we could do something like that (run over hurdles) as he has been off the track for nearly two years.
"He looks well and and is moving great."
Harrington's other National Hunt star is the admirable Supasundae.
The consistent gelding claimed his third victory at Grade One level in last season's Aintree Hurdle, before rounding off his campaign with a runner-up finish behind Buveur D'Air when defending his crown in the Punchestown Champion Hurdle.
"Supasundae is grand and doing his usual thing - it always takes a long time to get him ready," Harrington added.
"He might go to the Hatton's Grace at Fairyhouse in early December, or we might even wait until Christmas.
"We know he's a great spring horse."
