The Azure Marketplace constrains the available existing subnets to those that have a prefix size of
/25 i.e. 128 addresses or more. The prefix size must be large enough to be able to accommodate the largest size cluster/deployment that the Marketplace offering can deploy, and the prefix size chosen is the smallest that meets this requirement. The ability to dynamically adjust the prefix size based on the cluster topology to be deployed is not possible.
Fortunately, this is only a Marketplace UI constraint and not an ARM template constraint! It is possible to deploy a cluster using the ARM template from our GitHub repository directly, and select the existing virtual network and subnet that you want to use, using the networking parameters. Be sure to target a specific GitHub tag release for repeatable deployments.
This information has also been added to the documentation, and will be included in the next release docs.