"a healthy boy"

Translation: здоровый мальчик

[Neon_Iceberg]

"Здоровый" (masculine, single), "здоровая" (feminine, single), "здоровое" (neuter, single), "здоровые" (plural) also mean in Russian - "big", "huge", "strong", "forceful" These options are used in informal conversations.

[Ynhockey]

Not only that, but if you say "здоровый мальчик", it would mean "a healthy boy" mostly in the context of a baby. If you're talking about an older boy, it will almost always be interpreted as "a (physically) big boy" (unless you're specifically discussing health or something).