how to setup a default dimension for a sales order .
What is driving the dimension attribute for you? Customer? Order Taker? Item? Time of the Day? Day of the Week?
Default dimensions are stored on master data tables such as Customer, SalesPerson/Purchaser.
When you enter a customer on your sales header, the default dimensions from that master data table are copied across to the Document Dimensions that are attached to the Sales Header. These will in turn be copied to the Sales Lines.
When you post the sales order as an invoice, the dimensions will be copied to the posted document dimensions and also to the ledger entry dimensions.
You can set up the Document Dimensions on the Sales Header by keying them in through the application or programatically or by getting them to copy from related master data. What are you trying to do here?