you can sex snakes in three ways, Probing (v hard) counting the scales (really you need all the snakes from the same clutch so you can compare) and popping, this is when you apply a very gental pressure, either side of the anal plate, if its a boy the hemipenis will come out, if its a girl nothing will happen ( this is why you must be carfull, coz you'll probably think, Am i squeezing hard enough?) WRONG you could kill a female quite easly, this is why you should do it on a male so you know how much pressure is needed, also this method only works on hatchlings and juviniles...
The best way to messure a snake (i find) is to let it crawl along a flat surface againts a wall, and then messure the wall.
or you could use a piece of string, or just hold the head and tail and use a ruler (as long as your gental you probably wont hurt her..../him lol