Great aid for this is a little programme called HexGrid (though I am at loss trying to figure out where did I DL it from), you just load a picture, set the horisontal map dimansion (say 1500 meters), distance between hex flats (50 meters in this case) and the programme casts a hex grid over the map.
As for hex dimensions, I did a bit of exercise in geometry
if x is width of the hex and y is height of the hex row, then we know that
x = 50 meters.
Hex radius
r can be derived from right triangle with hypotenuse of
r and legs
r/2 and
x/2.
Then
r = (x/2)*sqr.root(4/3), that is 28,87 meters.
Hex row height,
y = 3/2*
r, that is 43,3 meters.