How to trim an Aggregator RSS feed?
I managed to do it using the Views module. I create a View of "Aggregator item" type.

In my view, I display each field I want to see, and for the "Aggregator:Title" field configuration, in the "Rewrite results" section, I checked "Trim this field to a maximum length". There are options to set the maximum length, to add or not an ellipsis (...), to add or not a "Read more" link... See the picture for more details (it refers to the "Aggregator:Body" field, but it works the same way).

Hope that helps,