Having a class for the dropdown could also help with styling it: https://feedback.telerik.com/blazor/1433654-component-specific-css-classes-and-ability-to-set-classes-per-instance. For example, when a dropdown class becomes available, you could set PopupHeight to "auto" to accommodate only a few items, and set a max-height rule for the dropdowns you want to limit when they could have a lot of items.
Regards,
Marin Bratanov
Progress Telerik