Hi there @Dario01
Not sure to understand the issue you're having. The length of the kick is adjustable via the LENGTH slider under the main window. This parameter is adjustable within the Pitch, Amp & Clicks 1/2/3 sections ( that length value is common to all these elements and it's given in milliseconds ranging from 50 to 3000ms . Next to that there's also a different length adjustment parameter for each of the Clicks samples on the right side of the GUI, those knobs will affect the endpoint of each click sample while the "start" knobs will control the start position of the samples, but that's something different to help adjusting the click(s) length to your overall Kick sound and not related to the final Kick length.
So the length of your Kick is not really control by the Midi notes length, but of course the speed of Midi playback will come into play : let's say for example that you have a very long 808 sub kick, if you're programming a very fast Midi notes sequence ( like 1/16 or 1/32 notes ) it might not be appropriate with the Kick sound length. An option to program such fast sequences ( for example to make rolls ) is to render the kick to audio and shrink the length using fades to adjust the kick length & avoid artifacts or clicks & pops during playback.
But again, not sure if I'm answering your question correctly and if I understood it well Feel free to detail what's you're trying to do and what's giving you issues.
Hope this helps