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From the home menu, click on:
Mods* --> Next --> Singleplayer --> Set Up Game --> Map Type --> Additional Maps
Here it should be listed if you scroll down a bit. Select the size map and the rest of the settings are up to you.
If the maps don't show, here, then simply unsubscribe to the mod and resubscribe and try again. This is because Steam mods are notorious for not installing properly.