It looks like you're new here. If you want to get involved, click one of these buttons!
How do I use the DRC length command to measure the linear length in microns of a particular layout for a particluar layer
DRC's length is an edge selector, not a measurement tool. DRC is based on layer operations, not numerical measurements. You can use "layer.perimeter" to derive the total length of all edges if this is what you want.