i would recomend buying a bowl or seperate tank for him
NO, it is NEVER ok to keep a goldfish in a bowl. Please never reccomend this. I understand if you didn't know that goldfish are unsuitable for bowls, so I'll tell you why:
As you stated, goldfish can grow over a foot long. The common goldfish, what terrorist_124 has, can easily grow longer then 1 foot. The fancy goldfish usually get to about 8-12", some even more. Can you imagine trying to fit a foot long goldfish in a bowl? It wouldn't be possible. You would be stunting the fish very badly, his body may stop growing but his insides (organs) won't. The fish will die a slow and very painfull death.
Goldfish produce A LOT of waste. And I mean
A LOT. More then any tropical fish. They would be producing way too much ammonia for a small bowl, you'd have to be doing water changes 2 times a day.
In the end, keeping a goldfish in a bowl is very inhumane, but you did suggest getting a seperate tank for him, which is great!
terrorist_124, a 20 gallon or so tank would work great. If you want to get him a buddy (which I highly reccomend, since goldfish are very social fish) you could go with a 30 gallon tank. You could even have a small shoal of either danios of WCMM (white clouded mountain minnows) or a small group of weather loaches, because they all prefer the cooler temps (like your goldfish does). Your goldfish will be much happier, he'll have his own "goldfish buddies" and the heat won't be bothering him. It will be also easier to feed him, since he needs to eat goldfish food, not tropical food.