I want them to fit again next year, so I'd like to get them a little big.
I don't want him to outgrow them this year, so I want to buy a bigger size.
この場合、次のような言い方ができますよ。
ーI want them to fit again next year, so I'd like to get them a little big.
「来年も着せたいから、少し大きめが欲しい」
to fit again next year「来年もまたサイズが合う」
ーI don't want him to outgrow them this year, so I want to buy a bigger size.
「今年大きくなって着られなくなって欲しくないから、大きめのサイズが欲しい」
to outgrow「大きくなって着られなくなる」
ご参考まで!