Designing a 6x64 Decoder
Feed the same three lower address bits to eight 3x8 decoders, whose outputs (8x8 = 64 bits) are your row select  lines. Use the upper three address bits as the inputs for a single 3x8 decoder, whose outputs select one of the 8 decoders at a time by controlling the enable input.  

An alternative is to use one 3x8 decoder for the upper three address bits, another for the lower three bits. You'd then take one of the eight outputs from each of the two decoders, and AND them together. There would be 64 such combinations, so you'd have 64 AND gates, each mapping to an unique address.