Hi, Welcome to the Forum Misterpotenzos !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
If you like to introduce yourself, to the community, go here:
http://lmms.io/forum/viewtopic.php?f=4&t=4480
and I want to learn this program
Perhaps my LMMS rookie-guide can interest you?
https://www.youtube.com/watch?v=hcUCFfQ3wn8
If you mean actual size of a note, then you must change that from the right end of the note. LMMS does yet not support note-length-changes from the leftwhen I make the melody, lmms doesn't make me reduce the size of the notes on the piano
where the size-change-icon of two arrow are shown
If you on the other way mean scaling the entire song-editor, you do that with your mouse-wheel, & modifier Ctrl -or you can choose a scaling factor in the dropdown Horizontal-zooming