well if the size is doubled so your original dimensions are (L)6 (W)8 (H)4 then turns into (L)12(W)16(H)8 so you plug them into the formula SA=(wL+hL+hw). It then becomes 2(16x12+8x12+6x16) you do parentheses first.
16x12=192
8x12=96
6x16= 128
192+96+128=416
416x2=832
so your final answer is 832