In the taskbar config dialog, add a spinner widget to specify the number of rows to use. Options would be automatic, 1, or 2 with the default being automatic.

When increasing the height of the panel, there is a threshhold at 38 pixels where the taskbar automatically reverts from one row to two rows for displaying the apps. There should be a config option to allow users to override that threshhold, similar to the config options provided in the virtual desktop pager applet.

Many users who increase the size of the panel likely do so to improve readability --- eyesight and mobility/dexterity issues. To have the taskbar revert automatically to smaller taskbar icons means they become harder to read or access for people with various physical issues, thereby defeating that reason for increasing the size of the panel.

Some folks might argue that with a setting of one row the task bar then quickly becomes crowded with the larger panel size. Probably not. Many users are not geeks or power users and do not run multiple desktops or dozens of apps at one time. Most users typically are single taskers with only two or three apps open. The likelihood of the taskbar becoming crowded is non sequitur because should the taskbar become crowded with the larger panel sizes, the new config option allows users to use the automatic option or specify two rows.

I struggled with whether report this should be an enhancement request or a bug report. I'm leaning toward the latter because I believe this creates a usability issue.