You should first do solubility test. Get the solvent in which the Schiff base ligand is very soluble in. Then also determine where it is soluble only in hot. Use a mixyure of these two solvents so that the compound will diffuse from the more soluble solvent to the less soluble one.
Not all Schiff base ligands show similar solubility. First examine the solubility of your ligand in all available organic solvents. Take the one in which it is least soluble and try to dissolve the maximum quantity by warming the solution. After obtaining a clear solution, filter it hot to remove any impurities and residue and then cool the clear solution and keep it aside. To get different size crystals choose a combination of two solvents in which it is highly soluble and least soluble. Make a concentrated solution of ligand in highly soluble solvent and added the second solvent in which it is least soluble until you get turbidity. Then add a few drops of the first solvent to get the clear solution. Keep it aside and you will get beautiful crystals. Use this method of any of the chemicals you want to crystallize. Good luck