It's a standard DIN. The Astra (new shape 98 onwards) actually has 2 standard DINs but they are not next to each other. Depending on your cars options, the CD holder/'small shelf with rubber mat' next to the heating controls is actually another DIN if you take it out.

I've had an Empeg installed in 2 Astras, a 99 2.0 SRi and curently an 02 Coupe Turbo in the 2nd DIN with the factory fit unit as Aux-in to the empeg via line level converters.