sdd:length
sdd:length is the literal datatype of a sdd:Section's fixed main-axis size — either a percent of the container's main-axis length, written "25%"^^sdd:length, or an absolute length in the same viewBox coordinate space as sdd:at/sdd:size, written "20"^^sdd:length (design spec §1). There is exactly one geometry unit system in the drawing standard; an absolute sdd:length carries no px suffix or unit marker of its own.
Absolute lengths never rescale when a node's overall size changes — a container that over-commits fixed sections clips them instead — which is the guarantee that gives fixed headers and footers a size that survives however the rest of the node grows (design decisions log). sdd:length is also the second half of a dual-datatype choice: sdd:size carries sdd:vector on a node and sdd:length on a section — the datatype disambiguates the same predicate's two shapes (design spec §2).