I'm not aware of technical limitations to size of neo4j-admin, just whatever your machine can handle. More power = faster load times but no file size limits that I know of.
I've used it to initialize a database of over 100 GB with billions of nodes & relationships. This was a MS-SQL db that I used SSIS to export the data as CSVs, transforming the data into a graph model instead of the originating star schema model. Think of neo4j-admin as doing a "bulk" insert, skipping the transaction and roll back logs and inserting data straight into the data file. You have to get your data formatted just right ensuring referential integrity and all that good stuff because as it imports it validates the graph structure, but it is fastest way to initialize a database.
To answer your second question, yes Neo4j can do pattern matching or community detection. I originally stumbled across Graph DBs when I was searching for pattern matching solutions and Cypher is tailored to writing queries to look for patterns. You'll want to read about Graph Data Science there's a whole section on Community Detection.
If you want to save yourself from buying expensive hardware or have the flexibility of scaling up or down, Neo4j has a cloud hosted service Aura that I'd check out before buying hardware.