Could easily be the size of the phb.
None of these worlds need a deep dive, and nowhere near that much crunch is needed.
The PHB covers the multiverse in 3 pages, the DMG does it in 25 Pages.
Guildmaster's Guide to Ravnica spends ~70 pages on the Guilds, and zooms in on the Tenth District for 23 pages.
A Planar book that spends ~70 pages on the Planes proper, and zooms in for ~23 pages of Sigil would be reasonable, and a major expansion to the DMG material.