You can enable Mesh Edit Tools addon.
Then, open sidebar in 3D View (N key).
Then go to Edit tab, open Mesh Tools panel, Edge Tools subpanel.
And press Set Edge Length button. Enter desired length and validate.
Without addon, you scale one edge until it has required length.
You can orient view to obtain y axis parallel to an edge. Then, you just have to scale to zero along this axis, individually, each edge that is supposed to be perpendicular to that axis.