I would not use a circle there, but instead use a semi circle, then run the bed to the left to about 3 feet or more from the house,the
Plant some dwarf shrubs against the house, or shrubs (yew, or Japanese hollies, Dwarf Hinoki Cypress ) that can be maintained low. then plant some color in front of the green backdrop.
Then maybe a groundcover, Ajuga? under the Crape Myrtle, to get early color.
And to answer your question 8 feet. the bigger the better.