I am not seeing that journals actually require documents to be published in the 21cmx11in format.
Looking at the submission guidelines for articles, it provides two preferred styles to download:
BioMedCentral_article (ZIP format) - preferred template
Springer article svjour3 (ZIP format) - preferred template
I downloaded the svjour3 one since the first one is specific to BioMed.
Looking at the svjour3.cls that comes with the zip, the relevant lines seem to be
Their fallback code is
As a side note, I have published with Springer (and with Online-only journals) and they, in my experience, prefer standard A4 format because even online-only journals want to generate .pdf files for download. Springer does list the standard TeX article class as a template download, too.
So not using the geometry package, one could set custom lengths and sizes using the built-in commands.