Generate custom sizes

I'm using a plugin (OneSignal) which requires additional smaller generated launcher icons so that they can be used in Push Notifications.

More specifically, I need 24x24 and 36x36.

Here's some more info:
https://documentation.onesignal.com/docs/customize-notification-icons#how-to-add-default-icons

Is there a way for me to generate custom sizes with flutter_launcher_icons?

If not, it'd be great if you could add such a feature, as well as suggest an alternative way in which I can add those sizes manually for the time being.