Dashborad will be sitting in your file so probably will be the same size as the file.
Dashboard is graphical representation of your data so it will take as much as the data you are representing.
Create a chart save it and the see how much your file "weight".
there is no default.
It depends of:
1. Nº Rows
2. Nº Columns, type and size
3. Distinct values from each column. If You have many distinct values on column the qvd will be bigger.