They can grow pretty big but take a good while to reach a large size. My mum has one that is about 18 years old and is about 2 meters, perhaps a little more.
Agree, Hibiscus - mine looks similar right now. Growth as @Aero84 says. Varieties of Hibiscus syriacus are often sold in garden centres etc., so the flowers are likely to look like one of these:
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