First, let me say that my reference for this is with HPUX systems.
I am guessing that by fragment you are asking about the byte size for the segments or extents. That depends on the size of your data the default size for most systems is around 4MB. If you have very large datafiles you can set this number higher, upto 256MB or more, but for normal data you should be okay to leave it at 4MB.
Inodes represent the number of files you can create. On some systems, this number is a fixed number that is set when you create a filesystem. IF you set this number too low, you will not be able to create a file even if you have space left on the filesystem.
On systems with JFS Online, I believe that it is a variable amount that you don't have to worry about running out of inodes. You should never have to worry about it with the more advanced vxvm filesystems.
I am not sure what you mean by allocation group size. Maybe you can explain that a little more.
Hope this helps.