I don't like DM's so was prepared to hate the bag !
However, it's leather ! which I am impressed by. ( unlike the Vivienne Westward one ) yes it looks small but I would need to get a ruler out of my cupboard to work out it's size :)
and it's leather - did I mention it is leather, and a sensible shape nothing too wierd and someone that wears DM's may be very happy with the brand name on it.
If you could give the receipt with it, or even better see if the company would do a gift receipt ?! then yes go ahead.
( as if the person really hates it then it can be returned and the money put towards a pair of DM's )
I think a chosen gift is far nicer than the flowers/chocs and gift card for M&S ( which may end up being spent on undies or food )