We actually do have a hidden preference that you can use to increase or decrease the width, you can try this by quitting Agenda and entering the following in the Terminal.app:
defaults write ~/Library/Containers/com.momenta.agenda.macos/Data/Library/Preferences/com.momenta.agenda.macos maximumNoteWidth -float 1100
Where the 1100 is the width of the note in pixels. The minimum is 450, and default 900, but you can see what works for you. Let us know!