@MikeZ87
normally, you can do it using "Edit Custom Style" (right button on component), but in ComboBox, in fact, the items is a "ListBoxItems", and there is not a property to change it in StyleBook...
that way, you can just right-click on ComboBox, and select each item and change the property on Object Inspector, or by code like this:
for var i: integer := 0 to (ComboBox1.Count - 1) do
begin
ComboBox1.ListItems[i].StyledSettings := []; // remove parent styles...
ComboBox1.ListItems[i].Font.Size := 28; // changing the properties...
end;
exists a "delay" before show new setting...