You can disable this feature by deselecting 'Size to content' checkbox in the Note Tab of the Notezilla Preferences menu since by default a sticky note is automatically sized to content.
Also, you can change the default sticky note size using Notezilla Preferences. To access Notezilla 's Preferences, right click on Notezilla icon and choose Preferences from the menu. Then click on the Note tab to set the desired size.
Watch this video to get more clarity on the feature: