People often have a hard time choosing the right size decorative lantern for the exterior of their home. It can be a little overwhelming at the lighting store. There are lots and lots of choices, often all crammed together. What I typically see is that homeowners tend to choose fixtures that are too small: when installed they look dinky on the house. Here is my recommendation of what to do when trying to select the right size fixture:
Before purchasing the fixture, see if you can borrow a sample or two from the lighting showroom to take home and try out. I would recommend selecting fixture that is one size larger than you think is the right size. The fixture doesn't have to be the exact finish that you're looking for, prior to ordering, it just needs to be put up against the façade and tested size-wise.
Some of these fixtures tend to be a little bit on the heavy side so ask a strong family member or friend told that the fixture in the approximate position it it will be installed while you run out to the street and take a look at how the fixture looks in proportion to the house. Try make a quick decision since, like I said, these fixtures tend to be heavy.
If the show room won't let you borrow a sample then take the measurements and create a template made out of cardboard. It's really worthwhile taking this additional step when making a long-term investment like this.