The N900 is compatible with (micro)SDHC, which has a limit of 32GB. That's why they recommend 32GB, because there are no SDHC cards with 64GB.
We can proceed in providing a microSDXC Ultra 64GB card as a replacement, however please not, that since the card is not supported by the specific phone, if the card has issues again we won’t be able to replace it again for you since you will be using it to a non-compatible device. We remain at your service should you require additional information.