Concerning the filesize storage sitefinity don`t limit you how much data will be stored and also you can choose to store the documents either in the site database, which I suppose will not be used as an option as having 3TB database is very hard to maintain, in the file system here is more information on configuring different storage for sitefinity media modules.
As I see not all files are MS Word documents and sitefinity search index can index the content of MS Word documents (PDF, .txt also) and only those documents will be targeted by the search for reading the file contents in order to add it to the search index. The operation for reading file contents is slower than the other search indexing operations in sitefinity and can take some time if the search index is reindexed for all files reading a content that is 30 mb for each document.
The reindex operation of sitefinity search index is not needed actually to add the documents to the search index.
When a document is uploaded its content is indexed as part the publish operation if the site have already created search index from Administration->Search and Indexes that is configured to search documents.
The re index (that can take long time for all documents) is needed if all documents have already been uploaded and the search index is created after their upload, reindexing documents will all already available content in the search index to make it available for search.
As sitefinity document search pipe is retreiving documents for indexing on small chunks in sitefinity 6.1.4300 and after a chunk of documents is indexed the next chunk is taken for indexing the handling of this large amount of files can be handled.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items