Al = 26.98 g/mol
Mass = 26.98 / 6.022*10^23 = 4.480*10^-23 g/atom
Mass of one atom = mass of one unit cell
Mass of one unit cell = 4.480*10^-23 g/unit cell

D = M/V thus V = M/D
V = 4.480*10^-23 / 2.70 = 1.66*10^-23 cm^3/unit cell
For a cube V = l^3 thus l = cube root of V
Unit cell edge length = cbrt(V) = cbrt(1.66*10^-23 cm^3) = 1.18*10^-8 cm