You're mounting the wrong device for one, sd0
c represents the
entire disk.. you should be using the letter indicated by the generated label, that will represent the partitioned portion of the disk.
To find the proper device node, examine the output of:
$ disklabel sd0
Now, finally.. the inode size error is something of a problem, OpenBSD's ext2 implementation only supports an inode size of 128 bytes.
A
patch does exist to fix this problem, but it hasn't been approved yet.. if you feel capable of testing it, reply to that thread with the results.
I hope that helps..