So. Your request is for Start8 to resize images to fit in the startbar?
Negative. (NO!, Absolutely NOT!)
The request is that you figure out what Windows 7 and Classic Shell are doing that allow start button height to be higher than Task Bar height.
In more technical terms. Start8 seems to be using the same display location top coordinate for the start button as for the task bar. THIS IS WRONG.
Start8 needs to use different window coordinates for the start button that allow a higher top position (and a bigger height), if the selected icon height is higher than task bar height. This allows the start button to have a total height higher than the task bar (of course the icon needs to use transparency/alpha channels so whatever is behind the icon in the area above the task bar will bleed through where appropriate).