At this stage, there is no way of adjusting the size of KPI cards. They appear at different sizes if they're a 'Series' type, as opposed to a 'Non-series' type. When the layout of KPI cards is set to vertical, and you have a 'Series' type next to a 'Non-series' it looks terrible.
I reached out to HighQ Support about this, and they advised it is running as designed with no intention of changing it. They suggested I raise this issue here in the forum to see if it gathers any more interest.
Thanks!