What's the size of the last Ripple ledger and the indices to search through it? What is the size of the current ledger and all indices required to efficiently search through it or query it? Also, how will this size grow relative to the number of Ripple users?

Side-question: what database technology is used by rippled to store the ledger?
Ledger 8470000 in JSON format (without inner nodes) is 115 MB large. You can get it if you issue the "ledger" API call on a rippled that you own - it's an admin-only call, as it takes lots of ressources to assemble it.

If you are asking about the size of the databases rippled uses in the backend, these are about 9 GB (ledger headers + validations archive) ~100 GB (transactions) and ~320GB (RocksDB node store, snappy compression enabled) on my node.

Rippled can utilize several backends, the most mature/recommended is RocksDB, a LevelDB adaption by facebook. In general rippled just needs a high performance key-value store for the largest chunk of data (nodes), transactions and other stuff is stored in SQLite databases.