How big should the upper teeth of a young child be?

Which tooth? . Are you talking about an adult or baby tooth? How old is the child? Adult teeth in upper anteriors are much bigger than the baby teeth. If that is what you are looking at then don't be alarmed. It is normal. But give me more information please.
6 mm x 11mm. If you we are talking about adult front teeth in a child , average size is 6 mm x 10 mm , when fully erupted . Primary front teeth , usuly under the age of six are about 5mm x 5 mm. Don't worry if your child's first set of adult teeth look big, they grow into them.
Tooth size. Tooth size is genetically determined. An upper front baby tooth should be tiny with space on either side. An upper front permanent tooth about twice the size of the surrounding baby teeth. All children should begin dental visits at eruption of the first baby tooth, and see an Orthodontist at eruption of their first permanent tooth. See a Dentist, a Pedodontist, or an Orthodontist. .
Small. I assume by "young" child you mean the first set of teeth. These should be tiny. With spaces between these baby teeth to allow space for much larger permanent teeth which begin to come in about age 6.