Answer: If you are asking about the hardy shrub hibiscus or Rose of Sharon (Hibiscus syriacus) which is a woody plant that grows into a large shrub or small tree you would want to plant far enough away from fences or walls for the plant to reach its full mature width. In general, if left untrimmed, these can grow to about ten feet tall (or higher) and up to about ten feet across.
If you are asking about the perennial flowers such as Hibiscus moscheutos with the dinner plate sized blooms, these can reach five or six feet tall and wide (depending on the specific cultivar.)
They all need full sun to do their best, so plant where they will not be shaded by a nearby building.
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