Is it safe to increase/alter the size of the wp_post.guid column from VARCHAR(255) to VARCHAR(2048)?

No, it is not safe to resize columns in WP core tables, or to alter them.

When you next update WordPress, the update process will alter the tables to the official table schema, truncating all your GUIDs and breaking them.

Instead, you need to contact the support route for the plugin so that they can fix this. They shouldn’t be reusing GUIDs to store URLs if the URLs are going to be long. This is a bug in the plugin.

Edit: There’s a ticket on Core Trac related to this, reproducing and posting that you can replicate it and it affects you will help, as will watching/starring it