How to find the size of an unmounted partition on Linux?

The command is:

blockdev --getsize64 /dev/mmcblk0p1

It gives the result in bytes, as a 64-bit integer. It queries the byte size of a block device, as the kernel see its size.

The reason, why fdisk -l /dev/mmcblk0p1 didn't work, was that fdisk does some total different thing: it reads in the partition table (= first sector) of the block device, and prints what it found. It doesn't check anything, only says what is in the partition table.

It doesn't even bother if the partition table is damaged, or the block device doesn't have one: it will print a warning that the checksum is not okay, but it still prints what is finds, even if the values are clearly non-sense.

This is what happened in your case: /dev/mmcblk0p1 does not have a partition table. As the name of the device shows, it is already the first partition of the physical disk /dev/mmcblk0. This disk contains a partition table, had you queried it with fdisk -l /dev/mmcblk0, it had worked (assuming it had an msdos partition table).

Try lsblk, it doesn't even require root:

$ lsblk -b
NAME    MAJ:MIN RM        SIZE RO TYPE MOUNTPOINT
xvda    202:0    0 34359738368  0 disk 
├─xvda1 202:1    0  1676673024  0 part [SWAP]
└─xvda2 202:2    0 32682016768  0 part /var/spool

The -b parameter tells it to output the size in bytes.

To get the exact value of total size of the partition run:

awk '{print $1*512}' /sys/class/block/mmcblk0p1/size