First flush diversion is a good idea to avoid capturing the first bit of a rain to wash the dust and dead squirrels off the roof. 10 gallons per 1000 square feet is a good rule of thumb. There are fancy gadgets for that, but a standpipe with a valve at the bottom seems to be the one that works. It ties into the gutter right before the downspout, so it has to fill before water can flow past it to the downspout.
A 6″ pipe holds about 1 gallon per foot. An 8″ pipe holds about 2 gallons. How can we make it look good?