If you want to do something at runtime but don't know how things are named you're out of luck. Before you begin at runtime do the same at designtime, then you see all the names and their relation in the ObjectInspector:
Add a stringgrid to the form.
Add some columns (Click at '...' next to "Columns", click "+ Add")
Select a column in the component tree above the property list in the object inspector.
In the property list find the node "Title", open it. It has a "Font", open it. Find the "Size" and change the value.
In total you have this:
- StringGrid1
- Columns[index] ("index" is the index of the column which you clicked, it is listed in the componenttree)
- Title
- Font
- Size
Write all these names separated by a '.' - and you got it!
StringGrid1.Columns[index].Title.Font.Size := something;