Order Line Details and Order Status Dimension

Hi All,

Am thinking best way to model my orders fact table. The grain is one line for every line item on an order. However due to the fact we need to track an order as it is raised, processed and despatched or cancelled we will use a status dimension and then add a new row for every change in status as well. I will then create a degenerate dimension for order number. However this is going to be a huge table with around 20 million rows per year and around 5 years of data. My questions are:

1. how do I calculate number of line items per order?
2. Is my design optimal for this volume of data?
3. will I need to use distinct count (I will be using SSAS) and is this optimal at this volume of data?

Any help advice very much appreciated.