Once you have switched your storage allocation method to manual, you can adjust the storage limits on a per-site basis. To do this, follow these steps:

1. From the SharePoint Admin center, select Active sites and a site that you want to adjust storage for.
2. In the information panel, under Storage limit, click Edit:

3. Enter a maximum storage value in GB. The minimum size is 1 GB, while the maximum is 25 TB (25,600 GB). Enter a storage consumption warning threshold value and ensure the Notifications slider is on to ensure the site owners are notified.
4. Click Save.

The threshold notifications are generated by a timer job that runs once per week, so you may want to implement a script to check the storage capacity more often. For a sample script, see https://docs.microsoft.com/en-us/sharepoint/manage-site-collection-storage-limits.