I've actually done a lot of calculation for hexagons lately as I'm making a couple of scrap quilts.

Firstly, hexagons are pretty big. A 5" hexagon (that is, a hexagon measuring 5" finished on each side) measures 10" point to point and 8.75" side to side, which is a decent size block. If you want to calculate dimensions for other hexagons, the formula is: Length = 2 x side and Width = 7/4 x side (or 1.75 x side). So for a top of 5" hexagons to finish roughly 72" x 84", the best arrangement would be either 8 x 8, giving dimensions of 70" x 80", or 9 x 9, giving 78.75" x 90".

Secondly, hexagons don't nest like straight squares, they have overlap between rows and columns (unless you are using setting triangles). To calculate the number of hexagons for a given top, you need to calculate it like you do blocks on point. For a top of Z blocks wide by Y blocks down, you need Z * Y + (Z-1)(Y-1). For example, if you are doing an 8 x 8 arrangement, you would need 8 x 8 plus 7 x 7, or 64 + 49 = 113.

If you are using setting triangles then the figures will be different but I can't do any calculations unless I see exactly how the blocks are arranged.