You may use this ways:
<RadioButton Width="80">
<RadioButton.Content>
<TextBlock>
<Run>First line</Run>
<LineBreak></LineBreak>
<Run>Second line</Run>
</TextBlock>
</RadioButton.Content>
</RadioButton>
<RadioButton Width="80">
<RadioButton.Content>
<TextBlock TextWrapping="WrapWithOverflow">Very very long text</TextBlock>
</RadioButton.Content>
</RadioButton>