You have to use python in the field calculator. The basic operations are listed here under the Geometry Unit Conversion section. The basic syntax for filling in length from lines is:
You can convert units with expressions like:
To get the average line length interactively,
right click on the Shape_Length field in the attribute table
and click statistics.
let's say, for example, that the average is 5000 meters
then you create a feature layer of the lines feature class, with a where clause:
"Shape_Length" < 5000
You can make the layer interactively in ArcMap by adding that
statement above as a Query Definition to the layer,
or inside or outside of ArcMap by using the Make Feature layer tool
and setting the statement as the where clause or Expression
Now use the layer as the input to Feature To Polygon
Only the lines of less than the average length (in our example, 5000 meters)
will be used by the tool.
Yes, instead of getting the statistics interactively, you can use the Summary Statistics tool
Yes, that tool produces an output table.
Yes, you have to look up the value of the MEAN for the field on which you requested statistics.
A search cursor will do that quickly (there will only be one record).
If you assign the value you have plucked from the table to a variable, you can use it in the subsequent where clause.
You never mentioned using model builder before, so I gave you the interactive workflow.
A Model Builder workflow will mirror the interactive one.