With no documents open; Edit > Preferences > Units & Increments > Keyboard Increments > Size/Leading:

This is a document-level preference and will only take effect in documents created after you set it in this way. Documents created before the change will retain the setting that was in place when they were created. For those, you'll have to change the increment with the document open, and it will use the new setting from then on. Note that you can even make it a half-point if you wish, (0.5 pt). Personally, I like all the increment settings to be significantly smaller than their respective defaults.