You will need to subscribe to the pack and then do the following.

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.

How do you choose this map for a single player game?