It's the "rectangle" 

This article indicate +- 100 "rooms" : https://www.rtbf.be/article/un-reve-pacifiste-aux-origines-de-linternet-le-mundaneum-dhenri-la-fontaine-et-paul-otlet-10945766

Here the plan with the "rooms" : https://www.flickr.com/photos/peacepalacelibrary/3113465938/in/photostream/ 

The book indiquate nearly 1 hectare and the "rectangle" is around 9000 m2

Some indoor pictures : https://search.arch.be/fr/rechercher-des-archives/resultats/inventaris/rabscan/zoekterm/%22palais+mondial%22/dao/1/eadid/BE-A0510_723341_806867_FRE/inventarisnr/I7233418068671350/level/file/scan-index/2/foto/510_2673_000_01350_000_0_7726

For the name the part "du Cinquantenaire" seams more descriptive of the localisation than part of the name but various sources use both.