Here is an extended solution for the problem of automagically labelling a polygon with the area as it is created. In the labelling dialog, use the Expression Based Labelling Engine (credit to Nathan) to implement this line
"Field ID" || '\n' || toint( $area / 10000) || ' ha'
The layer has a UTM (metres) CRS and so calculates the hectares on the fly. There is a concatenation to add in
which is a line break to make the label appear on two lines, also another to add the text of
ha. Field ID is the name or label for the polygon.
You can simplify the expression to calculate distance and display that. However it only displays once the feature has been completed, not whilst it is still being created which I think is the OP's interest. For me it is not much trouble to trial and error the editing of the feature to get the desired length.
(I'll have to admit I don't know any more than this, Otto showed me how to do it....)
This is for Lisboa 1.8, it will be different in trunk or older versions.
Where are the Expression-based Labeling options in QGIS?