You can use the length tool (the arrow button) on the OGF scale helper as a way of measuring lines quickly. If you want something more advanced, there’s a tool in JOSM that does it, although I do not use it myself.

In Potlatch - press E button, then draw a line, and you’ll see its length.

In JOSM the length of current segment (in drawing mode) or the line in general (when it is highlited) is shown in the bottom panel.