There're several ways to check the size of a raw device before creating a disk group in ASM.
Getconf
We can use getconf DISK_SIZE to check the size of a raw device.
[root@primary01 /]# getconf DISK_SIZE /dev/ASM_MGMT
204800
The returned size is in Megabyte (MB), therefore, the size of the raw device is 200GB.
Bootinfo
We can also use bootinfo -s to get the size information of the device.
[root@primary01 /]# bootinfo -s ASM_MGMT
204800
The returned size is also in Megabyte (MB). Please note that, we don't prefix /dev/ this time.