Assuming that the focal ratio of your telescope is f/6, its focal length is 8 x 6 x 25.4mm = 1219 mm and the magnification produced with a 10mm eyepiece and 2.8x Barlow lens is approximately 1219/10 x 2.8 = 341x. Mars currently subtends a mere 15.9" (its maximum diameter until 2016), which is only about 1/4th the apparent size of M57 (the Ring Nebula) and 1/113th the average apparent size of the Moon. Even at a magnification of 341x Mars won't be all that big but you should see a clearly discernible disk.
You may find this
site useful.
Dave Mitsky