I think we need to know why you want a car bigger than the Astra - is it to carry objects, or because you have long legs or broad shoulders, or carry passengers who get squashed, or because you prefer the 'feel' of a larger car?
Most MPVs are (roughly) mid-sized cars with big bodies on them, e.g. the Zafira is basically a big Astra, and the C-Max and S-Max are a big Focus/Mondeo. They might do if you want the pure space, but if you want it to feel a bit more 'premium' then I'm struggling a bit - the Lexus IS and Alfa 156 were smallish but nice-ish cars, but all cars seem to be behemoths now.