DBpedia defines datatypes for units and dimensions.

The page below shows a "hierarchy" of datatypes, although there's no such thing as subClassOf for datatypes (see #22). In PropertyMapping.unit you can specify:

Either a super-datatype, in which case the templateProperty is expected to have an explicit unit. Eg in Mapping_pt:Info/Futebolista:

{{PropertyMapping | templateProperty = altura | ontologyProperty = height | unit = Length }}

Or a specific datatype, in which case the templateProperty carries a pure number, which is interpreted as the provided unit. Eg in Mapping_en:Infobox_street:

{{PropertyMapping | templateProperty = length_ft | ontologyProperty = length | unit = foot}}

In both cases:

The number is converted to SI units (eg meters for Length)
In some cases, a specific property with a more appropriate unit is also created. Eg http://dbpedia.org/ontology/Person/height is in centimeters