Depending on the building, canopies can be really difficult to get right. They don't always work well with the elevation or frontage and can cause all sorts of issues with 'fitting in'. We've had a good look at our library and have chosen a few options that could be considered for your project. There is a mix of zinc, glass, wood, steel, cement and even a portico too.
I can hear you wondering if you actually need a canopy. You are right to wonder. Other than the obvious reason of for shelter it's worth considering that by having a canopy you automatically reduce maintenance by protecting your door from weathering. Hardwood doors in particular perform better over time when they are protected. A minimum depth of 500mm would be a good start, but if possible having a 1000mm canopy would be the best option.
A few things to consider when working on a canopy would be to be clear on the depth you have above the door and whether or not the style is too imposing or awkward on the elevation.
Whatever you choose for your building, a canopy or porch is a great way to protect your door and enhance its performance.