In the speed-bar of piano-roll, you have the dropdown Last-note

Click that, and chose any size you like!
Observe the option Last-note !
This mode is without doubt the one you use the most!
Is insure that the note you create has the exact same length as the previous clicked!
In taht way you can 'pick' note.lengths as you compose, directly from any note in the piano-roll!
The default length in std template, is the 1/16 p resize.
Read:
https://lmms.io/wiki/index.php?title=Co ... oll_Editor