This is a very young kumquat tree, only about six feet tall, but it has a bumper crop of fruit. It grows in the front yard of a neighbor. I don't know if she uses the fruit, personally I don't find it appealing raw, but I love kumquat marmalade. I haven't made it for years though there are lots of kumquat trees around and the owners would share fruit. No incentive, I guess.
The fruit will fit comfortably in my cupped hand. Although I didn't notice until I downloaded the photos to my computer there is a mayfly on the leaf. Perhaps that will give you a good comparison for size.
Here is a blog with a post about kumquats that I found interesting: