Currently, Opel has no entry in Europe's fast-growing minicar segment (light hatch) to compete with cars such as the Ford Ka or Volkswagen's forthcoming Up.
Mr Reilly said there has been a structural change in customers' habits with more people downsizing to smaller cars.
“In my view Opel needs a mini in today's market, that will be our top priority,” said Mr Reilly.
The decision could see Opel's 2004 Geneva Motor Show concept, the Trixx 3+1 seater (pictured top), stage a comeback, but we may have to wait until 2012 to find out.
Mr Reilly also said Opel will look at introducing battery-powered electric cars in addition to the Opel Ampera plug-in hybrid, which will have a range-extending combustion engine.
The Ampera, a sister model of the Chevreolt Volt, will eventually be built in Europe but will initially be imported from the US when it goes on sale in 2011.
Opel's smallest vehicle currently is the Agila, a small minivan which is built by Suzuki in Hungary with its platform sibling, the Suzuki Splash.