It would work like that normally yeah, you set the complete height for the combobox in expanded state. Wheter that is with too few or too much items.
With the size style in mIRC however you prevent it from growing bigger then the height you specify, as expected, however it does not prevent it to shrink down which when theres not enough data. Which doesn't happen in windows.
So making the height bigger has no effect in his case, he either has to choose to remove the size style which would cause the dropdown to grow (disproportional) or not support hsbar as you said.
