font size in header of read only and editable grid

Your understanding is correct you can't customize header in grid only values can be customizable.

Hello [mention:00e6688d68c542b99c8c455359d12cd4:e9ed411860ed4f2ba0265705b8793d05] 

Simple answer is No. You cannot adjust the size of label in the Grid Columns.

It is not possible for readonly grid. But it is possible to mimic such changes in editable grid. FYI, it has its own limitations and will be suitable only for certain scenarios

{
  a!gridLayout(
    labelPosition: "ABOVE",
    headerCells: {
      a!gridLayoutHeaderCell(),
      a!gridLayoutHeaderCell(),
      a!gridLayoutHeaderCell()
    },
    columnConfigs: {},
    rows: {
      a!gridRowLayout(
        contents: {
          a!richTextDisplayField(
            value: a!richTextItem(
              text: "Column Number 1",
              size: "LARGE",
              style: "EMPHASIS"
            )
          ),
          a!richTextDisplayField(
            value: a!richTextItem(
              text: "Column Number 2",
              size: "MEDIUM_PLUS",
              style: "STRONG"
            )
          ),
          a!richTextDisplayField(
            value: a!richTextItem(
              text: "Column Number 3",
              size: "LARGE",
              color: "ACCENT"
            )
          )
        }
      ),
      a!forEach(
        items: enumerate(3),
        expression: a!gridRowLayout(
          contents: {
            a!textField(),
            a!textField(),
            a!textField()
          }
        )
      )
    },
    selectionSaveInto: {},
    validations: {},
    shadeAlternateRows: true,
    borderStyle:"LIGHT"
  )
}