The medium maps in Phasmophobia are Bleasdale Farmhouse, Maple Lodge Campsite, Point Hope, Prison, and Sunny Meadows Restricted. That classification is listed in the Phasmophobia wiki map overview and medium-map category page.

Medium maps list

Map| Notes
---|---
Bleasdale Farmhouse| Medium 13
Maple Lodge Campsite| Medium 13
Point Hope| Medium 13
Prison| Medium 13
Sunny Meadows Restricted| Medium 13

A common point of confusion is that Sunny Meadows itself is large, while Sunny Meadows Restricted is the medium version.

TL;DR: the medium maps are Bleasdale Farmhouse, Maple Lodge Campsite, Point Hope, Prison, and Sunny Meadows Restricted.