Hi Sun riser,
Hope you are using batch management......Then proceed as follows....
Create characteristics called Z_PACK with allowed values 100L, 250L, 25L etc...with T-code CT04 and make this as required characteristic
Create a batch class like Z_PACK_CLASS with class type 023 through T-code CL01 and assign above characteristic Z_PACK in that.
Now assign this class to material in your case F00056 in classification view in material master in MM02..
When you do GR for this material system will ask you to select the value as characteristic is mandatory..
Check and revert...
Thanks
Kumar