I have used the SAP sizing guide in order to do a sizing for some SAP SSM10 implementations. I hope you can used it too , in my experience gives results very near to reality of a solution for an implementation.
Basically, you have to consider 3 primary issues:
1. Number and type of concurrent users
2. Number of cubes
3. Volume of cubes
Next an example of one of my estimations:
| # USERS | saps | user * saps |
Business User | 25 | 70 | 1750 |
| | | 0 |
Casual User | 25 | 378 | 9450 |
| | | 0 |
Power User Dashboard | 4 | 1445 | 5780 |
| | | 0 |
Power User Initiatives | 4 | 266 | 1064 |
| | | 0 |
Power User Strategy | 4 | 191 | 764 |
| | | 0 |
Power User Report | 4 | 827 | 3308 |
| | | 0 |
Power User Scorecard | 4 | 203 | 812 |