Customize not setting excerpt length

Hello,

I exported the blog from our site on Squarespace. I see that there is a field in the export <excerpt:encoded>. My problem is that although the import into WP seems to have been successful and all of my posts are present, the post excerpt seems to be set to a particular length that varies from post to post and which cannot be overridden by Cusotmize > Blog > Blog Content > Blog Post Content (set to show excerpt) and then adjusting the Excerpt length value. Is the problem the <excerpt:encoded> field in the records I imported into WP? If so, is there a way to ignore that field, or do I need to delete all the <excerpt:endcoded> fields before importing?

Thank you,

Jenny