Where can I see the length of a certain way element in the OSM data?

Pure OSM data only indirectly contains length information: The lengths themselves aren't stored anywhere but can be calculated from the nodes' coordinates. Therefore, you need a tool that calculates and displays this information.

OSM editing applications typically offer this feature:

• In JOSM, you can see the length of the selected way in the status bar at the bottom of the editor window (next to the icon)
• In iD, press Ctrl+Shift+M to toggle the measurement panel (alternatively click on "map data" in the right hand menu and tick "show measurements" field). Now click on the way that you want to know the length of.

Maperitive can show you lengths of ways:

1. Load the OSM data from a file using the "load-source mydata.osm" command
2. Find a way using some kind of search criteria, for example "find way[highway=footway]". This will find all footways.
3. List the results using the "list-results" command. It will show a list of all matching ways, their IDs and lengths in meters.
4. You view each individual search result using the F3 (find-next) and SHIFT+F3 (find-prev) keys.

1. Visit https://openstreetmap.org/edit and pan to location of interest.
2. Ctrl+Shift+M (Toggle measurement panel.)
3. Click on the way that you want length of.

OSM data contains only the geometries stored as nodes, ways and relations. If you want to obtain the some statistics on an element, you will need to work with tools like Postgis or specialized program.

Postgis 1.5 has a new geography type which can be used to return numbers like length, area in meters. You would need to use osm2pgsql to import your data and then create a geography column based on the geometry column created by osm2psql.

This method is quite complicated and should mostly be used for more advanced users.