So according to the MM a Young Dragon is a "Large" dragon.
According to the size chart in the DMG (Pg 248) an average large creature is 10ft tall. It also shows us that the average Huge creature is 20ft tall and the average Gargantuan creature is 30 ft tall.
On Page 249 of the DMG that the creature fits within a 2x2 grid, which is 10ft x 10ft. So a large creature should normally fit into a 10ft x 10ft x 10ft cube.
Using the MM stats for an Adult Dragon we can estimate the wingspan for a Huge creature. The Wing Attack states:
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon...
So we can surmise that for a Huge Dragon each wing is no longer than 10ft long.
Using the stats for an Ancient Dragon we can see what happens to the dragon's wingspan when we go from Huge to Gargantuan:
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon...
So moving up one size level, from Huge to Gargantuan, increases the maximum length for each wing by 5ft.
Using this information, combined with the DMG size chart we can infer that for each 10ft increase in height the wingspan of a dragon increases by 5ft long.
So bringing this all together, for the Young Dragon the wings should be 5ft long each. This gives us a total wingspan of 10ft + body width for a Young Dragon.