The following code will change the font size in all charts in the active
workbook:

'=======================
Sub ChangeLegendFont()

Dim chObj As ChartObject
Dim ws As Worksheet
Dim ch As Chart
Dim lFont  As Long

lFont = 6 'set the font size

For Each ws In ThisWorkbook.Worksheets
    For Each chObj In ws.ChartObjects
        chObj.Chart.Legend.Font.Size = lFont
    Next chObj
Next ws

For Each ch In ThisWorkbook.Charts 'chart sheets
    ch.Legend.Font.Size = lFont
Next ch

End Sub
'================