With Loki needing roughly 108GiB[1] and Prometheus at least[2]
116GiB[3], 200GiB sounds like a good starting point.

[1] increase(loki_ingester_chunk_stored_bytes_total[1w]) / 7 * 90
[2] https://www.robustperception.io/how-much-disk-space-do-prometheus-blocks-use
[3] (rate(prometheus_tsdb_compaction_chunk_size_bytes_sum[1w]) /
    rate(prometheus_tsdb_compaction_chunk_samples_sum[1w])) *
    increase(prometheus_tsdb_head_samples_appended_total[1w]) / 7 * 365