たとえば、
\documentclass[paper=a5,papersize]{jlreq}
のように、jlreq.cls で a4 以外の用紙サイズを指定し、papersize オプションを指定しても、エラーにはなりませんが、無視されます。つまり、出来上がった PDF を Adobe Reader で読んだとき、用紙サイズを確認すると、210 mm × 297 mm のままです。
\usepackage{bxpapersize} を加えると、サイズが反映されます。jlreq.cls では、papersize オプションは使えないということで、よろしいでしょうか。
タイプセットすると以下のように警告が出ます。
papersize オプションは使えませんね。
C:\Users\tony\Documents>platex hoge
This is e-pTeX, Version 3.141592653-p3.9.0-210218-2.6 (utf8.sjis) (TeX Live 2021/W32TeX) (preloaded format=platex)
restricted \write18 enabled.
entering extended mode
(./hoge.tex(guessed encoding: Shift_JIS = sjis)
pLaTeX2e <2021-06-01>+1 (based on LaTeX2e <2021-06-01>)
L3 programming layer <2021-06-01>
(c:/usr/local/share/texmf-dist/tex/latex/jlreq/jlreq.cls(guessed encoding: UTF-8 = utf8)
Document Class: jlreq 2021/05/28 jlreq
(c:/usr/local/share/texmf-dist/tex/latex/xkeyval/xkeyval.sty
(c:/usr/local/share/texmf-dist/tex/generic/xkeyval/xkeyval.tex
(c:/usr/local/share/texmf-dist/tex/generic/xkeyval/xkvutils.tex
(c:/usr/local/share/texmf-dist/tex/generic/xkeyval/keyval.tex))))
(c:/usr/local/share/texmf-dist/tex/latex/everyhook/everyhook.sty
(c:/usr/local/share/texmf-dist/tex/latex/svn-prov/svn-prov.sty)
(c:/usr/local/share/texmf-dist/tex/latex/etoolbox/etoolbox.sty))
(c:/usr/local/share/texmf-dist/tex/latex/base/ifthen.sty)
(c:/usr/local/share/texmf-dist/tex/latex/jlreq/jlreq-helpers.sty(guessed encoding: UTF-8 = utf8))
jlreq guessed engine: platex
(c:/usr/local/share/texmf-dist/tex/latex/lm/lmodern.sty))
(c:/usr/local/share/texmf-dist/tex/latex/lm/ot1lmr.fd)
(c:/usr/local/share/texmf-dist/tex/latex/l3backend/l3backend-dvips.def)
LaTeX Warning: Unused global option(s):
[papersize].
(c:/usr/local/share/texmf-local/tex/platex/tony/hoge.aux) [1] (./hoge.aux) )
Output written on hoge.dvi (1 page, 252 bytes).
Transcript written on hoge.log.
papersize オプションは使えませんね。
C:\Users\tony\Documents>platex hoge
This is e-pTeX, Version 3.141592653-p3.9.0-210218-2.6 (utf8.sjis) (TeX Live 2021/W32TeX) (preloaded format=platex)
restricted \write18 enabled.
entering extended mode
(./hoge.tex(guessed encoding: Shift_JIS = sjis)
pLaTeX2e <2021-06-01>+1 (based on LaTeX2e <2021-06-01>)
L3 programming layer <2021-06-01>
(c:/usr/local/share/texmf-dist/tex/latex/jlreq/jlreq.cls(guessed encoding: UTF-8 = utf8)
Document Class: jlreq 2021/05/28 jlreq
(c:/usr/local/share/texmf-dist/tex/latex/xkeyval/xkeyval.sty
(c:/usr/local/share/texmf-dist/tex/generic/xkeyval/xkeyval.tex
(c:/usr/local/share/texmf-dist/tex/generic/xkeyval/xkvutils.tex
(c:/usr/local/share/texmf-dist/tex/generic/xkeyval/keyval.tex))))
(c:/usr/local/share/texmf-dist/tex/latex/everyhook/everyhook.sty
(c:/usr/local/share/texmf-dist/tex/latex/svn-prov/svn-prov.sty)
(c:/usr/local/share/texmf-dist/tex/latex/etoolbox/etoolbox.sty))
(c:/usr/local/share/texmf-dist/tex/latex/base/ifthen.sty)
(c:/usr/local/share/texmf-dist/tex/latex/jlreq/jlreq-helpers.sty(guessed encoding: UTF-8 = utf8))
jlreq guessed engine: platex
(c:/usr/local/share/texmf-dist/tex/latex/lm/lmodern.sty))
(c:/usr/local/share/texmf-dist/tex/latex/lm/ot1lmr.fd)
(c:/usr/local/share/texmf-dist/tex/latex/l3backend/l3backend-dvips.def)
LaTeX Warning: Unused global option(s):
[papersize].
(c:/usr/local/share/texmf-local/tex/platex/tony/hoge.aux) [1] (./hoge.aux) )
Output written on hoge.dvi (1 page, 252 bytes).
Transcript written on hoge.log.