A ramp will most likely need to be much much longer than the porch steps. These specifications can help you:
Be sure to follow ADA Guidelines and install a handrail.
The ADA guidelines are intended for those who use a wheelchair. They recommend no more than 1 inch of rise for every 1 foot of run, because it's difficult to push oneself up a steeper incline. Even an electric wheelchair should have no more than 1 & 1/2 inch rise per foot of run. The above guideline means descending 24 inches needs a 24 foot ramp. As long as you can walk you can have a steeper ramp that will also be shorter, but a steep ramp should also have a non-slip surface. The photo in this link is NOT ADA compliant, but it is easier and safer to use than the original stairs: https://wumc.files.wordpress.com/2009/12/dscn2291.jpg