membrane cfd

hi.i working on the modelling of one hollow fiber membrane with large length.i run the model sucsesfully. but when i want to select a cut plane the system shows the 'out of memory -when xmesh computing' error ,
since the length of fiber is very large.one solution for this issue is that i change the real dimensions of fiber to dimensionless one(real dimension/a real number).but i dont know how to do it :( can any one to help me?
king regards.