I was creating a RAID array and got this error: mdadm: /dev/sda1 is too small: 0K
mdadm: create aborted

Of course sda1 is not too small, both partitions sda1 and sdb1 are identical in size:

Disk /dev/sda: 2000.3 GB, 2000398934016 bytes
255 heads, 63 sectors/track, 243201 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1               1         974     7823623+  83  Linux
/dev/sda2             975        3407    19543072+  83  Linux
/dev/sda3   *        3408       60801   461017305   83  Linux
/dev/sda4           60802      243201  1465128000    5  Extended
/dev/sda5           60802      231009  1367195728+  83  Linux
/dev/sda6          231010      243201    97932208+  83  Linux

Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1         974     7823623+  83  Linux
/dev/sdb2             975        3407    19543072+  83  Linux
/dev/sdb3   *        3408       60801   461017305   83  Linux

 The solution is simple, use partprobe to read-read the partitions

Just run:

partprobe

And it will work, but the weird thing is that these partitions were created a few reboots ago.  This is obviously some weird quick with the older mdadm version.