(a) Electronic configuration of Li is 2,1 and that of Na is 2,8,1. Since sodium has more number of shells, thus a larger size.
(b) E.C. of Cl = 2,8,7
E.C. of S = 2,8,6.
As sulphur is placed before chlorine in the periodic table, so it has a bigger size and its tendency to gain electrons is lesser than Cl.