Exclusion limit for Sephadex LH-20 is 4-5 kDa, and that depends on solvent (due to different swelling). So it seems to be good for your molecule. You should take into account that swelling of this Sephadex in DMF is higher than in most other solvents, so its pore size is relatively high in DMF. But I still think that you can use this support with DMF. I attach a reference containing useful data, including the table of swelling volumes in carious solvents.

Thank you sir.

You can calculate exclusion limit by packing the column. It will be the volume that is excluded by beads. It can be determined by Total volume of column-void volume.