The duration is not determined by the space of the note. If you use samples, generally you let the sample end by itself, but with other synth generators or with looped samples, you control the note duration by adding a note off.
Note off is added with the button "1" over the keys (i.e. not the one in the numpad, in case your computer has it).
You will see then a "off -- " entry instead of a note.
You can also change the amount of lines of a beat, in case you need more precision, or also you can use the delay command ( FDxx, see tweakings.txt in the docs folder ) so that the note (or noteoff) .
<[JAZ]> Pa pi pa pa pa pi pa.... ;ĚD