How to reproduce:
Set the AutoSpellCheckControl property of the spell checker and use it in a DPI-aware application on a system with an increased scaling. Type an incorrect and notice that the context menu will not be scaled.

Workaround: 
Public Class RadForm1
    Sub New()

        InitializeComponent()

        'TextBox
        Me.RadSpellChecker1.AutoSpellCheckControl = Me.TextBox1
        Dim dpi = NativeMethods.GetSystemDpi()
        Dim checker = TryCast(Me.RadSpellChecker1.GetControlSpellChecker(GetType(TextBox)), TextBoxSpellChecker)
        checker.DropDownMenu.PopupElement.DpiScaleChanged(New SizeF(dpi.X / 96, dpi.Y / 96))

        'RadTextBox
        'Me.RadSpellChecker1.AutoSpellCheckControl = Me.RadTextBox1
        'Dim dpi = NativeMethods.GetSystemDpi()
        'Dim checker = TryCast(Me.RadSpellChecker1.GetControlSpellChecker(GetType(RadTextBox)), TextBoxSpellChecker)
        'checker.DropDownMenu.PopupElement.DpiScaleChanged(New SizeF(dpi.X / 96, dpi.Y / 96))
    End Sub
End Class