Retrieval-augmented generation depends on both the retrieval method and the units over which retrieval operates. Yet retrieval-unit choices remain underexplored for Thai legal documents, where language and document structure challenge common chunking assumptions. We introduce a 2,000-question Thai Royal Gazette dataset with single-hop and multi-hop tasks. The dataset annotates gold evidence as document character spans, which permits unit-independent evaluation. We compare 19 configurations across chunk sizes, boundary strategies, contextual chunks, atomic facts, whole documents, and retrieval methods. We evaluate them with span recall and precision, an Evidence-Fit judge, and pairwise judgments of answers generated from retrieved evidence. Contextual 200-word chunks with hybrid retrieval provide the strongest practical balance of evidence coverage, answer quality, and query cost. Across configurations, downstream scores correlate strongly with span recall, while span precision shows little association. These results favor retrieval designs that first recover complete evidence, then control cost and localization.