Well, with most notes just hold the cursor above the right end of the note in the piano roll and find a spot where the cursor will look like this: <->
Then you can start dragging the note shorter or longer.
There's a bug with really short notes, like if you first click on something in the Beat/Bassline Editor and then try to elongate it in the piano roll. To solve this, take one of the menus at the top-right of the piano roll called "Note Length", select "1 bar", click a note somewhere, click that note again, and change the option back to "Last note". You should now be able to make long and resizeable notes. And avoid those really thin notes.