I have looked through all the specs and it is not listed anywhere for this device. I just purchased this model but I am skeptical to use it without knowing the amount of storage in the device.
My older Roku Ultra LT keeps running out of memory and I finally added a 64k card that I hope will eliminate the issue for that device.
Wikipedia says 4GB.
https://en.wikipedia.org/wiki/Roku
I have no proof, but also no reason to doubt that.
That's around 60,000 times more than a 64K card has (if you really meant K - even if you meant M, it's still way more.)
Do you mean a 64 GB (gigabytes) card?
Make sure you format the card and that the card is one that is compatible with the device. Apps usually are 50 to 100 MB (megabytes) in size. For reference, depending on how many apps you have, 1 Gigabyte is equal to 1000 Megabytes.
I would also suggest doing a system restart or unplugging the Roku for 30 to 60 seconds to help clear the cache of the Roku device as that may help your memory issues as well.
If your "old" Ultra LT is model 4662 from 2019, it has 1 GB system memory and 512K channel storage.
All Ultras since the 4800 in 2020 (including Ultra LT 4801 and current Ultra 4850) have 2 GB system memory and 4 GB channel storage.
Source: Wikipedia chart https://en.wikipedia.org/wiki/Roku#Feature_comparison