That's a misunderstanding on your part. The fs_size_fixed option means that you cannot change the size file system inside the volume. You can change the size of the volume itself however. To understand what I mean, take any of your snapmirrored volume and run "vol status -b" on it. It shows three columns: the volume name, the size of the volume in 4k blocks and the size of the contained file system in 4k blocks. The value for both the volume and file system columns will probably be the same.
Now try to grow the volume using the vol resize command, and run "vol status -b" again. Notice that the two columns now have two different values: the one in the file system column is the same as before (because the fs_size_fixed option is turned on), and the one in the volume column is larger than before.
If you grew the source volume as well (where the fs_size_fixed option is not set and thus the vol resize command resizes _both_ the volume as well as the file system within), running the snapmirror update command will update the destination's file system size to match the source.