Vocab Size

The vocab size can be changed in the config via experiments.spm-vocab-size. The vocab is shared between the source and the target languages. The vocab is built by SentencePiece.

In “Finding the Optimal Vocabulary Size for Neural Machine Translation”, the researchers tests vocab sizes between 500 and 64,000. A general value table depends on the vocabulary size.

Corpus Size	Optimal vocab size
4,500,000	32,000 - 64,000
1,000,000	8,000
500,000	4,000 - 16,000
30,000	1,000

See section 4 for their conclusions. https://aclanthology.org/2020.findings-emnlp.352.pdf

The larger the vocab, the slower the training and the inference will be as each token in the vocab contributes to the amount of probabilities that will need to be generated when making a token prediction. This is a trade off on the quality of the translation, and the performance.