I agree with Brian that the Hong Kong kumquat would be a good variety to bonsai. Another good citrus variety to bonsai would be a procimquat. The fruit is tiny, about one-half the size of the Hong Kong's fruit, the leaves are small and the tree frequently fruits from seed in the very first year. Swimmingfree, if you want to stick to an actual kumquat the nordsman's fruit is oblong as is the nagami, the meiwa's fruit is round. Both are approximately the same size. Lastly, I think a Myrtifolia orange, a dwarfish tree with thornless branches and small, very closely set leaves, would also be a perfect tree to bonsai. - Millet