This is a common question and asked daily...i hope this will help you in the future.
The astra mk4 can take big wheels without any arch work and with a low drop. If you are willing to modifiy the arches massive wheels are possible.
16'' ...... these are fitted to the astra sri as standard so will fit fine although they can look a little small still. If you wanted to fit aftermarket wheels in this size these are the golden rules:
Maximum width to fit without mods on a 40mm drop is 8'' The offset needs to be between ET 40-49
17'' ..... Again these are fitted to the Gsi as standard so again no problems!! For aftermarket wheels this is the size range you need to stick to:
Maximum width to fit without mods on a 40mm drop is 8'' The offset's can vary though depending on width.
8'' ET 45-49
7.5'' ET 42-49
7'' ET 38-49
18''....These are big wheels that can be fitted to the astra with no mods as long as you get the size correct, if you dont you may have to roll the rear arches as room at the rear is tight the bigger the size wheel you go.
Maximum width to fit with a 40mm drop is 8'' The offset's can vary though depending on width.
8'' ET 45-49 With this width if you lower anymore than 40mm the car will need to have the rear arches rolled.
7.5'' ET 42-49 This is the perfect size wheel and if you get the correct size you can drop it upto 60mm with no arch work.
7'' ET 38-49 This wont really fill the arch that much but does mean you can go a little lower on the offset.
19''.... As with all cars any size wheel will fit if you are prepared to do the work. 19's do fit but wheel size is not only very important but also very rare. Room on the rear arch is the main problem with fitting 19's. To fit 19's this is the size wheel you will need, so that minimum modification is requiered with a 40mm drop:
7'' or 7.5'' ET 42-49 these are very rare though and will require you to roll the rear arches and relocate the rear bumper bracket. You will still get rubbing with 4 people in the car but it wont be too bad.
Any bigger will require serious work and a lot of modification. It will also impare the handleing and driveabilty of the car.
In Genral a 3 door hatch has the least room, with the 5 door, saloon and estate having a fraction more room due to the design of the car. Coupe's and convertables have even more room which means you can go for slightly different offsets but the best are listed above for all models.