I found a german ebay site which offers a similar product (too bad without picture) which claims to be 100% compatible with the 5800XM (and therefore with the n900) without being bigger in size.
I'll just buy one and write down my experience asap i'll get my n900

Here is the actual product: http://cgi.ebay.de/++-Power-Akku-LiP...QQcmdZViewItem
It claims to be 100% compatible with the original battery: