I am a real beginner with QGIS 2.8. I am able to generate a contour map from a raster DEM, but the contour map is very 'busy' because the land form is pretty flat. There are lots of short contour loops that clutter up the output which would be easy to remove if there was a way to select all contours greater than a certain length. QGIS is able to determine the contour length if I inspect a particular contour line, but this is a 'derived' value, and I do not find a way to use this derived value as a selection criterion.
There are several way for that
If you just want to select line shorter than 500 you could use
$length <500 (or
$perimeter <500 if you're working with polygons) as an expression in the select by expression (the 500 is the length in the layer CRS unit, if you want 500m made sure to use a projected CRS whose unit is meters)
Alternatively you could calculate a length field in the attribute table using
$length as expression (you could either create a new field (no auto-update if you edit the line but the value are added to your layer) or create a virtual field (auto-update on edit but the value only exist in this specific QGIS project, also usefull if you don't have edit rigth and can't add a new field).
After calculating the lenght you could select or symbolise by attribute as usual