This Nuspec reference explains the description and summary text fields: https://docs.nuget.org/create/nuspec-reference
In regards to the summary it says it is
A short description of the package. If specified, this shows up in the middle pane of the Add Package Dialog. If not specified, a truncated version of the description is used instead.
Does anyone know the max length of the summary, description, and how many characters from the description will be used if the summary is not provided?
There is no maximum length of the summary or description. If no summary is used then all of the description is used.
However what gets displayed is based on the size of the Manage Packages dialog, the size of characters used in the text, whether the row is selected and the Install button is showing, whether you are using Visual Studio 2015 or an older version. So there is not a defined number of characters defined as the maximum length.