Ok, report then what you did get. Because it is the same size as the original battery, I highly doubt you won't get any more than the original BL-5J 1320 mAh +/- some 10%.

There is no empty space in the original BL-5J battery, it fills the N900 battery slot completely. So only way to get more capacity in that same space would be to use some other type of chemistry than Lithium-Ion. And then it becomes a question could N900's charger suipport this new type of battery.

If you really need bigger battery, Mugen 2400mAh battery with bigger back cover is the only proven solution.