The other answers are technically correct, but not real-world correct. Here's what you need to ask the business:
What time horizon am I aiming for? In your case, you're looking for a 12-month number.
During that time, will we be archiving data, or keeping all data? In some businesses, you're allowed to (or required to) only keep a certain amount of data, like the last 12 months. In that case, you'll need to figure out the data growth (which the subsequent questions will answer) but then back down to the last rolling 12 months. You can't just say, "Right now that amount of data is 100GB," because if your data volume is growing, then the last 12 months is growing too. The time amount might be constant, but the data is not.
Will we be adding additional users? For example, the business might be growing into new territories or acquiring new customers. If they double the user base, then in some cases, the data will start doubling as well.
Do we expect the business volume to grow? If you're tracking sales on a web site, for example, and you start running Super Bowl or World Cup ads, your data volume can hit the hockey stick growth curve.
Will we be adding additional functionality in the app? If the app suddenly starts storing images, this will dramatically affect database size.
Will we be adding data from another source, or logging new data? If you start capturing web site clicks, or in a data warehouse, adding additional sources, then data volume will grow.
Will developers or DBAs be performance tuning indexes? If you're going to let people create indexes, you can easily double (or triple, or quadruple) the size of your data depending on how overzealous they get.
And as long as you're asking these questions, you should also ask if performance is expected to stay the same, degrade, or get better. I like mapping out the projected growth on a line chart, and then comparing hardware and staff training investments over that same timeline.