My problem is I can't change the width of the label in form view. I can give the group a colspan of 4 but the label just takes the aprox 100px space and then the input field takes up everything else. The only way I managed to stretch the label is by adding a long string to the group tag.
<group col="4" string="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed d.">
<field name="field_with_long_label"/>
</group>
Thanks Hilar! Your way worked... using style in xml doesn't work but with the class it works. Sorry can't convert your comment to the answer, got to little karma.
i can