Oh dear - this is a hard question to answer without more details. Clethra ranges in size from a small tree to a dwarf shrub of just 2 to 3 feet. You mention \"delavivi\" in your question - is that a variety name? if so, I am not familiar with that variety nor are my researchers. I do love the Clethra shrub however as it does well in light shade, has wonderful white blooms that attract butterflies, and thrives in slightly acidic, moist soil. Treat it much like a rhododendron to grow it to top size. In a spot with more sun and less water it will stay much smaller. This plant is also called Summer Sweet or Pepperbush and there is even a variety with pink instead of white blooms called \"Pink Spires.\" If you can find more information about the name of which clethra you would like to grow, I can be of better help in determining the size. A safe estimate for most varieties would be four feet tall by five feet wide - it tends to grow out more than up. Keep growing - especially new plants! Marianne Binetti