Using their real life equivalents as a size reference, here is roughly how many other regions can fit inside the Paldea region.

From left to right, top to bottom:

• Sinnoh – Hokkaido (DPPT/BDSP)
• Alola – Hawaii (SuMo/USUM)
• Almia – Hokkaido's tail (Ranger: Shadows of Almia)
• Lental – Galapagos Islands (New Pokemon Snap)
• Fiore – Okayama (Ranger)
• Johto – Kansai (GSC/HGSS)
• Kanto – Kanto (RBY/FRLG)

This is only a rough comparison, with sizes eyeballed using this website.