In design view, select the tab control itself -- not any of its pages, or
any controls on those pages -- and open its property sheet. On the Format
tab of the property sheet, you can set the various font attributes,
including size. These attributes apply to the tabs.
In design view, select the whole tab control (not just one of it's
pages).
By default, Access names the tab control "TabCtl0", "TabCtl1", etc.
Set the Font Size to whatever value you wish.
All the tab page labels will be the same size.