Hi,
as I don't know the backround of you application data I maybe just can give some more generic hints.
-> For good performance you dimension table shouldn't be larger than 10% of fact table.
-> If possible never have n:m relations in your cube. As far as I understand you star schema additional fields in the dimensions are only attributes of the key which is normaly a n:1 relationship and is ok.
-> For Coupon table I'm not sure. Maybe it makes sense to split if it is a large dimension to make use of line Item dimensions. This also depends on the key of the used InfoObject.
-> Be aware about the possibilities of having attributes in the dimension or as navigational attribute.
Regards,
Peter