Takeshi Ideriha complained that there is a mixture of Size and size_t
in dsa.c and corresponding header.  Let's use size_t.  Back-patch to 10
where dsa.c landed, to make future back-patching easy.

Discussion: https://postgr.es/m/4E72940DA2BF16479384A86D54D0988A6F19ABD9%40G01JPEXMBKW04