My sources tell me that Microsoft has released a document called "Axapta Hardware Guide For Environments Less Then 100 Users".
It's a word doc and will give you recommendations for
- Axapta Database Server
- Axapta Object Server (AOS)
- Axapta Application Server
- Axapta Enterprise Portal Server
- Axapta Batch Server
You should ask your Axapta partner for a copy of the document.
If you will have more that 100 users, you must enter some data in an Excel sheet which will then be processed by Microsoft (ask your Axapta partner).
Today, I also found in Harish Mohanbabu's Blog something about sizing. Check if out here: http://www.livejournal.com/~harish_m/