There are 2 different length parameters, one in the ARP section, one in the NOTE section.
As mentioned previously, in order for the arp to trigger, the NOTE length must be sufficiently long. So, by setting the NOTE length suitably short, and then playing with the envelope for that step, you could achieve what you’re looking for.
Cheers !
To be a bit more “academic”, the NOTE length sets the length of your arp, whereas the ARP length sets the length of the notes WITHIN the arp.
Hope this makes sense 